The ADO bus arrives in Corozal around 4:30 am, so you'll have a couple of hours to kill until the Thunderbolt departure time at 7am. There won't be much open at that hour. The vendors at the market next to the bus terminal probably won't show up until 5 or 6 am (I've never been to the market at that time of day) -- you can buy food there, and there are places to sit and eat. The Thunderbolt pier is about 500 metres from the bus terminal -- easy to walk unless you have a LOT of luggage. If you go by taxi, you'll need two -- BZ$5 or $6 per taxi. Maybe you could ask one of the minivan drivers to take everyone in one vehicle.
The one-way Thunderbolt fare is BZ$45 per adult and BZ$25 per child, but I don't know their age limit for the child fare. The Thunderbolt trip is about 2 hours.
The thing I'd be concerned about is the Thunderbolt departure is sometimes cancelled in the low tourist season.
If you continue to Belize City on the ADO bus, you'll arrive around 6:30 am and should be able to get on one of the boats departing at 8:00 am. The water taxi terminals (there are 2) are about 1 km from the bus terminal and you'll probably need two taxis (BZ$6 or 7 each) unless there's a minivan taxi available. Water taxi fare is US$15 or USD17.50 depending on water taxi company. The water taxi trip is about 1.25 hours. If you miss the 8 am water taxi or it's full, the next departures are at 9 am and 10:30 am according to the water taxi websites.
If you want to get the water taxi from Chetumal to Ambergris, you'll have to arrive in Chetumal by 2 pm in order to have enough time to get to the pier and buy a ticket. If you can't get there for the 3 pm departure, you'll have to spend a night in Chetumal. This water costs US$40 or $50 per adult (there are child fares). Another concern -- there is only one water taxi per day now -- if it's full, and you havent reserved seats, you might not get on.
The links to all the water taxi websites can be found on the BelizeBus site...http://belizebus.wordpress.com/