my personal opinion - altun ha is very very uneventful trip. the smallest and least interesting of the 6 mayan sites i've seen.
i can suggest a full day trip to Lamanai with Tanisha Tours (Daniel is THE BEST!!!!).
http://tanishatours.com/we did Lamanai with our 2 sons in August, and tho it is a long day, we had a BLAST. we saw lots of wildlife (including manatees, howler & spider monkeys), and learned bunches.