I don't know of any of the major packages that do this but I'd recommend you book seperate flight and accommodation, that way you pay a £ per night rate whether there are 2 or 4 of you in the room and you only pay for what you need on the aircraft. Thomson, first choice mytravel jmc etc will charge as child prices on flights up to 16 years and virgin, ba etc start charging for adults at 12years old,so this may make a difference, you may have to pay for a taxi transfer at the other end but this shouldn't cost you any more than about $40 to get to kissimmee/Idrive.
there are travelodge type hotels along idrive/kissimmee that start at about £20 per room per night if you're on a budget.