Combining A Baptism With 1st Birthday

Help! I have such a large immediate family that I decided to combine my son`s first birthday with his (long overdue) baptism celebration. The problem is, I don`t want my relatives to feel obligated to bring 2 gifts-1 for baptism and 1 for bday, how do I express that on an invitation (not to mention what type of invitation should I mail out??!) without sounding rude? and also, is it wrong to invite them for an open house serving cake and coffee in the middle of the afternoon? We`re trying to save on food needing to invite 90 people. Thanks