Roo,
Just got back from a week in the Marhaba Club with my wife and two small sons (5 and 3).
My initial feedback would be DO NOT rely on the Thompson holiday rep (Anita) for anything helpful. She and her predecessors had many complaints and have received more from us. If you are not going with Thompson then don't worry.
The hotel itself is nice enough. Reasonably clean. Any repairs (we had a blocked sink) handled immediately. The concierge more knowledgable and helpful than the rep and the main swimming pool is faultless. The guys on the door will do anything for you but would be glad of a small recompense ( I confess we did not oblige). The Marhaba Club is in a complex with a few other hotels that all share this pool. We went out and about on a daily basis so don't know how full the pool was through the day but by 6 it was empty (closes every night at 7.00pm -maybe that's why !). The beach is large and many umbrellas but the sea was rough when we were there (check the weather before you go).
TIPPING: some would say the most annoying part of the holiday. You are almost expected to tip every single time someone is helpful (apologies for any offence to anyone else reading this). However, the guidebook we got said that chambermaids are not expected to be given any tips, so we didn't. the only tips we actually gave were for the concierge on leaving since we had to get rid of all TND before leaving the country. A few 100millemmes should be fine (current xchng 1GBP=2.5TND approx). one guy at the airport wanted to push my suitcases to the check in desk. I politely said no thank you and he was starting to get shirty because, obviously, he wanted some money out of it. Not everyone on Tunisia is the same I hasten to add. We met some very nice helpful people when trying to find our way around Tunis.
SUGGESTIONS: for things to do with a 14 year old boy, from the Marhaba Club I would try the Sahara Expedition (which we did not go on but heard great things about) which takes you on a 3 day trip. The food is OK, if you like chips, salads, meat variety, solid breakfasts and nothing too fancy ( we stayed half board). Any other trips would depend on your interests.
Happy to talk more if you like
regards