I have this game for Kindle Fire. It's very cute and I have a lot of ponies now.
HOWEVER, this game can be a huge money hog. So, yes, set password protect your device so that you (or a little one) do not accidentally spend money on this game.
Seriously... It would take $500 in REAL money to get the amount of gems it would take to buy Princess Celestia ALONE.
The Kindle Fire version is buggy. You can sometimes add friends, but you still cannot go into another friends Ponyville to leave a treasure chest. So if you do play this game, I suggest that you use an iphone or similar tablet, and not an Android.