We have a kindle fire that the kids (4 and 2) use, although it wasn't originally bought for them.
The parental controls are pretty good - we have set up child profiles for them, which allow them to access apps and books that we've specifically selected as ones we consider suitable for them. You can set time limits so the child profiles will only work between certain hours, so say not before 9am or after 5pm, and set total time limits for the day, so they can only play on the tablet for anywhere between 15 mins a day to unlimited time. You can also set varying time limits for books and apps.
There's plenty of child friendly apps available from the Amazon appstore, ranging from purely entertainment value to more educational ones. Although if you're looking at the educational ones, we've found it's best to sit and do those with the DC to avoid them randomly jabbing the screen until they hit the right answer.
Obviously it's not great for a small child to do nothing but play on a tablet all day, but I don't see a problem with half an hour here and there. It's all about moderation.
And my DC enjoy doing plenty of things that don't involve technology. Today so far, we've had building a wooden train layout and driving trains on it, about 20 stories read (using real books, mostly the ones we borrowed from the library last week), making scones, and no requests for any tablet time at all.