Hi Kim,
It's not a silly question. My daughter has a spica cast on for hip displaysia (leg casts that go to the waist) and I wondered the same thing when she had her 6 month immunizations.
The nurse put them in her arm, I think they may be able to go in the buttocks but that area in my DD was quite restricted as well due to the cast. Apart from a little lump where the needles went in (which are still there slightly after a month) there were no issues with them going into the arm.
Monica