My 5 year old son and 3 year old daughter went with me to see this. It was great. The opening was a little slow for them (they expected to see action instantly based on the previews) but it unfolded nicely once I explained it to them. Excellent animation with fun, lovable characters (at least the good guys) and a "bad" guy alien who was easy to "boo" without being too scary made for great fun.
The movie's premise is kind of silly but you have to accept that going in. My whole family went (2 years old and up). It was fun as long as you put your logical mind on hold. Just sit back and enjoy the ride. Answered the slew of questions from my kids about "could that happen" and such. Good clean fun for an afternoon watching a thrilling adventure.