Every child & family are different, so I can only say what I'd do for my family. Take my opinion for what it's worth. Maybe, nothing.

Most kids want to celebrate their birthday on the actual day or very close. I would celebrate for those who didn't have a birthday fall on your trip dates, on their actual birthday or the weekend before (Mon., Tue., Wed. birthdays) or after (Thurs., Fri. birthdays). That would only leave those who had a birthday fall on the trip dates. The first birthday is a big deal. I'd make sure the baby had their own cake. If the soon to be 2 yr. olds are twins & their birthday falls on your trip, you could do one cake. They're probably used to it. If they're not used to sharing a birthday, I'd try to get something small for each of them. It doesn't have to be a big cake. Some of the bakeries have smaller options that would probably work for them. Just make sure to bring candles. If your birthday falls on the trip & you'd like to include the other kids on that cake, that would be great. I would only do that, if they had celebrated their birthdays individually also. The kids in your group may be different from the kids I know, but I don't know any that don't expect their own cake for their birthday, with the exception of the twins I know. FWIW, you could get several podium cakes as cheaply as one elaborate cake. While a podium cake wouldn't be my first choice, I find it preferable to the kids not having their own cake, if that's the only one they'll get. Mini cakes would also be an option. You just wouldn't get as much cake to share with the group.