From my understand he was actually next to her supervising.
The reason he was charged was because he only allowed her on HIS facebook account with him supervising. Because, you know, shes seven, and her own account would have been kinda stupid (plus dont you have to be 12 or something to have one in the first place)
Those games have two sides to them. The 'free side' where your using the ingame currency and doesnt cost a penny. But users typically have the option of using real life money to 'fast track' themselves if they cant be bothered making the ingame currency or want to get ahead of their friends.
Unfortauntly since he was logged onto Paypal (probably through facebook, the idiot) and his daughter will have seen 'do you want to buy XXX for £xxx' and she'll have thought yeah, and HE will have though it was ingame currency, when it wasnt.
This is not something that can just 'happen'. The girl WILL have had to click an option saying she wanted to buy. Unfortauntly she is seven years old and her dad is stupid.