This is a good game, with tonnes of levels. I love the feel to it, the various "pets" you can choose to put in the tank for the next level to get ahead. I did eventually finish it, but it took me months.
If the kids are going to playing it, I'd suggest a gaming mouse - there is alot of alien shooting action, and it tires the mouse buttons out a bit.
The aliens make it frustrating, especially when they eat all your fish, but the cute fish, great graphics, understanding gameplay, the concept and the challenge to get on to the next level, get you there in the end.