A lot depends on what hotel you are staying at and if you are all inclusive or not.

And it's rather like asking how long is a piece of string. Transport (buses, taxis, trains etc.) are extremely cheap, though taxi prices need to be 'negotiated'. or insist they put the meter on, so it is easy to get around. If you decide to eat out, once again prices are very reasonable and most restaurants/cafes have their prices listed outside so you won't get any nasty shocks. Just noticed that you will be in Sousse at the Marhaba Royal Salem A/I so let us know what you and your family are interested in doing and we may be able to give you a better idea.