Well I might be in the minority, but I think this little boy needs to realize that the world doesn't revolve around him. Sorry!!! By putting that candle on the cake, your hubby was reinforcing the little boy's behavior - I won't say bad behavior - maybe more like undesirable. And the boy should have learned you don't stick your fingers in somebody else's cake - especially twice. I believe the parents are at fault tho - IF he behaves at home, then he needs to be taught that he has to behave at other people's homes too.
Of course this is just my very old fashioned opinion.