Sorry that no one has answered your post. We have spent a total of three months on Maui in the last 5 years and I have seen very few taxis. There are some but my guess is that the rates are pretty steep. Without a doubt you can rent a subcompact for a day for less than the taxi fare. Make sure yo allow plenty of time because during rush hour the road from Lahaina to the ocean center, aquarium etc. at Ma'alaea can get pretty slow.
Why do you want to go there? If for the Aquarium, you can't duplicate that in Lahaina but if for a whale watching trip or a sailboat ride or something, you can do that just as well from Lahaina. Honestly there is not much in either place specifically for three year olds but there is plenty in Lahaina to entertain a reasonably curious child.
Landlocked in Denver, but cruisin every chance we get.
Polynesia, Carib. '86
Cr. Odyssey, Scandinavia, '91, 30 Day S Pac. 2002
Crystal Harm, Aust., N.Z., '94
Royal Odyssey, AK,'96
Old Cr. Pr. Canal, '97
RCCL, Carib, 1998
Volendam, Car, 2000
Ryndam, 35 day S. Am., Antarctica, '03
Is. Pr., Canal, 2004
Statendam, 34 day China, Japan, AK '06
Cr.Pr., Carib. 08
Eurodam, Atlantic, Med. '10