Well, if it is already booked and paid for, it's a bit late now to worry! Thomson's brochure states "cruises are most enjoyable for children between the ages of 3 and 16" and "cruise ships are not really suitable for babies and toddlers". So I doubt that they particularly make provision for them. However I'm sure the staff will be helpful. (You also have the long flight to worry about!).
We have been on the destiny three times ,staff are so friendly, and very good with the children, there is also a play rrom for the children with trained staff, we have been on thomson ships for the last 7 years and love them .