Children can get into difficulty by inadvertently running up bills when playing games on-line. Some games which are initially free to download are actually just a trail program. These are either time limited or have limited content, to continue playing or to download the full program involves payment.
Some games are financed through advertising, within the game there might be advertising for a real-life product, or the whole game might be designed to promote particular products or brands.
To prevent unexpected expense it is best to set rules at the outset regarding the downloading of software and apps.
You may well find it useful for the family as a whole to have a set of positive rules to reinforce your family values when on-line.
A sample of such a set of rules can be found here.