Without being too obvious, offer to take the kids camping, skiing, etc. some weekend.
On the way home, drop the wife off with kids at Dairy Queen and explain to them you have to run home quickly to do something, but you go straight to the friends place. Run screaming to their door explaining the kids went missing, but you've already got the authorities working on it. Let them panic and freak out for a while, then retrieve kids before it gets out of hand.
Proceed to give them the ol'...PSYCHE!!!! and explain that knowing their sense of humor, you thought they' get quite a kick out of it!
