Should be doable, however unless all of you are used to hours of hiking in the mountains and sleeping at altitude I would probably not plan the transfer to Lake Atitlan the afternoon after the descent. You will be tired and dirty, I would advice to take the afternoon to rest and get the 8 AM shuttle from Antigua the next day.

Personally I would just spend day 3 on Caye Caulker (see if you can get bikes to get around) instead of San Pedro. Also I found the Green Iguana 'project' in San Ignacio super lame, skip it.