Many current apps and online games offer extra services – sometimes you need them to get to the next level in the game – which you can buy with real money. It is important your children know this and realise full well they are spending actual money.
In-app purchases on your device can be restricted as follows:
On your Android-device open the Google Play Store-app, tap Menu > Settings and enable Require authentication for purchases.
On your iPhone or iPad go to Settings > General > Restriction and disable In-app purchases.