Strapping the kids in their car seats could be difficult, you usually need 2 hands.

Depending on how high up his arm the plaster is, and the position that his arm will be in he may be able to lift using his shoulder more than his lower arm. Can you younger son walk or stand up to help out?

Within a couple of weeks he should be able to work out most things, it's just the initial time that will be most difficult.