Since all King games are free to download and free to play, they offer in-game purchases instead to monetize the development, maintenance and support. For this reason the Piggy Bank and all other sale offers require a purchase to claim the Gold Bars.
All purchases are fully optional since you can complete all levels without any purchases.
Not only for children, but for all, it must be
free. Because you're playing for the piggy bank. I do not see and then
pay for it. That would be 2x in one. Not fair.