As I understand it, your whole family will be certified this summer. May I assume that they will not have been diving in the ocean before they go to Ambergris Caye?
I'm going to assume that your entire family will enjoy the certification process and will enjoy taking up diving. As long as they are doing it for themselves and not just to please you and your son. All that said and if all that is indeed the case, I don't think you will have any trouble keeping them happy to dive most of the holiday. As long as they are warm enough in the water and on the dive boat, they will be very happy and you may find that they didn't think a week was enough time.
Please be sure that everyone gets in a refresher dive shortly before you leave for San Pedro.
Other than that, I side with Chloe. Don't split up your holiday. You only have a week. You can find lots to do even without diving.