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No parent wants to be faced with a black mouldy nozzle on their child's reusable water bottle, but they're a magnet for bacteria and germs, and if the bottle isn't washed properly then it will most likely need replacing sooner rather than later.

We recently tested 12 kids' water bottles and part of our investigation involved giving each of them a good wash.

Not only did our tests show us the hardest to clean areas on each kids' bottle, but it actually surprised us how difficult some are to clean.

Find out what we learned about keeping your little one's reusable bottle mildew and odour-free.

Do you need a new kids' water bottle for the next school year? Head straight to our kids' water bottle reviews .

The best kids' water bottles are as simple to clean as putting them in the dishwasher, and we would recommend doing this every day or every other day.

But you need to make sure the bottle is stacked properly so that the high pressure water jets can reach every part of the bottle, particularly around the nozzle.

You should consider putting the cap of the bottle through the dishwasher twice, once while it's face up, and once face down.

You'll also get better results if you select the dishwashers hottest setting.

Searching for a new dishwasher? Take a look at the best dishwashers in our tests.

As you can see in the gallery below, the spout is the problem-area for most bottles.

A large collection of images displayed on this page are available at https://www.which.co.uk/news/article/five-ways-to-stop-bacteria-building-up-in-your-kids-water-bottle-aoBVG3g1qlPD

For bottles that have too many nooks and crannies to be cleaned properly in a dishwasher, or if you simply don't have one, you can get better results with a good old fashioned sink wash.

Make sure to use very hot water and washing-up gloves, then you can get to work with soap and a brush or sponge.

Take special care to clean carefully around the nozzle and cap, and rinse through the inside of the straw, as this is where dirt tends to linger.

In our tests, a thorough sink wash was enough to sufficiently clean some of the kids' water bottles, but not all of them.

Go to our washing-up liquid reviews to see which to buy and which to avoid.

It pays to break the bottle down into as many of its individual component parts as you can, for easier access to all the small places where bacteria can linger.

Some kids' bottles have spouts that can be removed and taken apart and we'd recommend doing this to get into each and every corner.

Where possible look for a bottle with a simple design as that means it will be easier to clean.

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We used this bottle cleaning kit from Amazon in our tests and we think it's £8 well spent.

Don't forget to brush through all of the straws and spouts to remove germs, and also brush the plastic part of the bottle cap and inside the bottle.

Many of the kids' bottles we tested were impossible to properly clean without using the small brushes that came with this kit.

Soaking the bottle in Milton sterilising solution for at least fifteen minutes once a week or a fortnight is a great habit to get into.

While it won't do a good job of removing any existing black mould that might have grown in the bottle, it's the best way of preventing mould from building up in the first place.

You can buy Milton solution in bottles, or the most effective option is to buy tablets or powder, to which you simply add water (£1.50 for a pack of 28 tablets) and these are available at most major supermarkets and pharmacies.

The bottles that impressed in our tests have a simple design and easy to access parts to help you keep bacteria and germs at bay.

As well as being quick and easy to clean, the best kids' water bottles are durable, leakproof and easy to use for little hands.

We conducted three different cleaning tests on each bottle:

Dropping in sand: Each bottle was rolled in a sandpit and then examined to see how much sand got stuck in and around the bottle cap.

We checked how easy it is to brush off and whether any bottles ended up with sand in the drink.

Dishwasher: We smeared the inside, bottle caps and drinking spouts of each bottle with greasy Vaseline and then washed each bottle in a dishwasher on its hottest setting.

Vaseline glows bright under UV light, so each bottle was illuminated with black light after it had been washed to show the spots that were missed on each bottle.We used Vaseline in our cleaning tests, because it glows bright under UV light.

Hand wash: We repeated the process of dirtying each bottle with Vaseline, and then thoroughly cleaned each bottle in a sink with hot soapy water.

If necessary we also used a bottle cleaning kit to reach every part of the bottle. After cleaning we checked for dirty spots with the UV light.

Hand wash: We repeated the process of dirtying each bottle with Vaseline, and then thoroughly cleaned each bottle in a sink with hot soapy water.

If necessary we also used a bottle cleaning kit to reach every part of the bottle. After cleaning we checked for dirty spots with the UV light.

Reusable water bottles can take quite a beating in your child's book bag or backpack, or generally through just being dropped on a regular basis.

We dropped each bottle from the height of 50cm, 10 times when it was full and 10 times empty.

The worst bottles have over complicated and flimsily parts which broke during this test. The best bottles only had superficial scratches.

You can't test kids' water bottles without involving some kids.

We observed a five year old and a three year old drinking from, filling, and tightening each water bottle and rated each one for ease of use.

A three year old, five year old and an adult filled each bottle with water and then put it on a paper towel for an hour to see if any liquid escaped.

While the adult was able to safely secure all of the bottles, this wasn't the case for the children in the test, who found it more difficult with some bottles than others.

Lockable lids are a good feature to look out for. They might just save you from a soggy bookbag further down the line.