Dear Hannah
You don't know how much has saddened me this message. I was looking forward to see yourself and spend this time with you. However I'll help you and your family to ejnoy your holidays in Formentera.
 So, one time over there you should go to the most representative local aglomerations like Sant Francesc, Es Pujols or La Mola. Many times at week  there are an artesanal market in La Mola. It'd be an interesting alternative if you wanna join mediterranean culture and bought some awsome souvenirs.  
On the other hand, Es Pujols is the home of intimate restaurants, bars and pizzeria's with delicious savories: something good for everyone. However, there are some nice little shops. But talking about serious shopping a tourist in Formentera should visit te boulevard in Sant Francesc.   I think you can find there evrything necessary for a tourist. If you visit Sant Francesc don't forget to do a walk around the village and see the local church, for example.
 Also resting on the awsome beaches you can do a lot of atractive and exciting activites as windsurfing, kitesurf or beach volleyball. 
 I think in this period the weather won't make you surprises: you'll find sunny beaches and clear sea water; how must to be. 
 Best reguards,
Michal

Michal Kolinski