You asked

How do I avoid costly bills from kid's apps?

We have listed some of our tips from our mums on how to avoid being faced with any unwanted charges from downloading apps or from in-app-purchasing:
Play the game with your child when it is first downloaded: This will help you understand how the game works and if there are any hidden costs in the form of in-app-purchases.  This way you can explain to your child that these involve real money and are not part of the game.
Buy a game outright: There are many games which you can buy outright for between £1 and £2 and while it might seem preferable to select the ‘free games’ these low cost games don’t generally feature any additional or hidden costs.
Only buy from brands that are trusted and always ensure to read the app reviews posted by other parents before downloading any game, free or otherwise.
Download a parental control app. If you are concerned that your child may be able to access adult content while using an app then download a free parental control app such as KidsPlace (android) or AVG Family safety for iPhone. These apps will transform your phone or tablet into child-friendly environment by blocking any adult content and by preventing your child from downloading any unnecessary material such as apps or in-app purchases.
Speak to your child: It’s important that you give your child guidelines for playing these games. You should explain the conditions he needs to agree to before he will be allowed to play the game. You should ensure that he understands that while he can play the free aspects of the game, it is too expensive to purchase any extras for the games.
Use a gift card:  If there is a particular game that your child wants to play for a special occasion you could purchase an iTunes gift card instead of a credit card which will help avoid unwanted costs.
For the parents of teenagers who wish to make purchases on their own device, but who do not have their own bank account, using an iTunes gift card instead of your credit card will help avoid unwanted costs.
Play games directly on trusted websites instead: Many mums are happier to allow their child to use a laptop or PC to play games directly on websites such as Nickelodeon or Disney. The games are often educational and are completely free.
Social media sites also have hidden charges: Social media websites especially for children such as Moshi Monsters or Club penguin are technically free to join but full membership which allows children access to special areas of the site requires a monthly fee. Some of our mums recommended purchasing a 1 monthly membership card for special occasions. These membership cards can be purchased in stores or online.
Never use the same passwords: It’s important that when selecting a passcode on your phone that you opt for one that isn’t used anywhere else in the house or one your child has access to.
If you are considering giving your child their own phone, call your mobile phone network and register it as a child’s phone. You should speak to them about what services you would like blocked.
Finally, if it has happened to you, call your mobile provider and explain the situation, they may refund you as a goodwill gesture. 

More questions

How to avoid the costs associated with mobile phone apps for kids, from downloads to in-app-purchases.
We cannot wait for the new Muppets movie come out! We have been fans of The Muppets since we were kids and are now really excited to share them with our own children. On vacation in Los Angeles, Walter, the...
‘A Monster In Paris’ comes from the makers of ‘A Sharks Tale’ and is a fun, family movie about a flea who turns into a hulking monster. A wacky inventor, his camera crazy best friend and...
Read our review on the Xbox Kinect
Check out the trailer for Puss In Boots
Read about our favourite PopCap games
Read our review of InnoTab from VTech