How can I make a mold of my son's foot?

I'd like to make a casting of my 1 year old son's foot (just a footprint will be fine) just as a sort of keepsake. Unfortunately, all the mold materials I'm finding require the object that's being replicated to sit in the mold material for a period of time (not so easy with a 1 year old's foot). Is there something I can use that would allow me to just quickly press his foot into it? I'd like something that will show a lot of detail if possible.