Kindle fire / amazon /milkshake

My father in law bought ds a kindle fire a couple of years ago. The first years subscription to kids stuff was free, but we found we couldn't download videos (useful to keep him entertained in the car) without having a full amazon prime subscription (£7.99 per month I think).
Now the first year has passed so we are paying an extra £1.99 per month for the kids content on top of the £7.99. But a load of his favourite content has no moved to "milkshake" (another £2.99 per month).

I'm a bit confused about whether we need to pay for all 3 of prime, kids and milkshake. If we stop paying for kids presumably all the games and books will stop working.

And are there any other good kids tablets? With the kindle fire I do like that we can give him the tablet knowing he can't access anything inappropriate (or spend any more of our money) but there's still plenty of things for him to do.