I have played with 5 year olds so I know your problem...

When my brother DMed for a group of 4 boys, all under 10 (including me; this was a long time ago ) he made up a houserule called "Sillies" (I know, stupid name). If you were acting disruptive, he would give you one. If you got three, something would happen to your character, say, lose a magic item. If you got 9, you would be removed from the mission. This was all worked into the plot...he was really good at improvising.

This tactic works, but don't carry it too far (and name it something else). Don't use it if people are joking around, use it if they are being truly disruptive.