To reiterate:
1. Start with the windows. If you are stuck with old single pane windows, I will almost guarrantee you that you are losing the majority of your heat through the windows. Adding something as stupid as saran wrap will dramatically increase your thermal resistance at the window - walmart / kmart etc sell kits for insulating your windows. It can be ugly - but effective.
2. Weatherstrip the doors.
3. After this, if you don't see any improvement, then I'd start thinking exotic. You're situation is a little different due to babies and birds, so you really can't just drop down to igloo temps and wear a sweater. Now, you could tie the bird to the baby, thus allowing them to both conserve heat ....
