How about taking them to Ardastra Zoo and Botanical Gardens?? It's a short cab ride away (about 10 minutes or so) and you can arrange a round-trip rate from your driver (be SURE to agree on a price BEFORE you get in the cab and include a "partial payment on the way back" clause if you can) and have him come back for you in an hour or two (won't take any longer than that - it's fairly small but quaint).
Be sure to take time to watch the flamingo show - they have trained flamingos that march around to an old man who shouts unintelligible commands to them (at least for humans, that is - the flamingos seem to understand what he's saying). I've been there twice and each time saw school groups there so it's fairly popular with the little ones.
Like I said, it's not huge so short attention spans won't get bored from being there for too long. They have kids rates and adults are only around $10 or so - they've got a website with exact prices, just google it. If you go, watch out for the sleeping pig!!
Nancy G.
Fantasy to Bahamas - 10/04
Fascination to Key West, Playa Del Carmen (Hurricane Wilma took out Cozumel!! ) - 11/05
Inspiration to Cayman and Calica, MX - 11/06
Victory to Puerto Rico and the Virgin Islands - 11/07
Sensation to Bahamas (with my dd for her b-day!) - 1/08
Inspiration to Cozumel and Grand Cayman - 12/08