Even though you can't cashout profit from these games I think it's still not advised for kids, because they can become addicted. Until the kid wins the desired reward, he won't stop investing money in the game, especially because these online games' rewards are offered for limited time, so it must be achieved fast. Personally, I wouldn't let my children invest money in online games besides traditional membership packages.
These betting features are made in a way to force you to keep spending tons of money without limits in order to collect all the game's items. And doesn't matter how much money you put on it, you will be never able to reach 100% of the game collectibles.
I am not yet a parent but the thought of me that a child may possibly engaged in a game or in an online game with a money involve is beyond my imagination but possible. The kid don't know what this money involved game is all about and this is where the role of the parents should be call of. This is where the thought of fear of mine coming from for the child even if its not my own child but introducing technology to a child is inevitable so the one thing that a mature person besides a child while she/he grows plays a vital role is guiding them and teach them wisely.