Europeans used to be able to buy alcohol in some supermarkets, in Sousse and PEK at least, but recently they seem to have tightened up the rules, and with the rise in religious fervour I think it may be even harder now. There will be no problems getting a drink in hotels but I wouldn't count on being able to buy alcohol in the supermarkets as I certainly couldn't last time I was there at that time of year. As there are quite a few of you going I would recommend that you each take your full quota of duty free.
Incidentally although restaurants and cafes will be open during the day you may be asked to sit inside during daylight hours (i.e. whilst Moslems are supposedly fasting). You may also find that taxis are rather scarce for about an hour each evening when the drivers go home for their long-awaited meal.