Place it at the exact beat of the fail in your edit, after a cartoon slip for a physical gag, then follow it with Nelson's Ha-Ha! when you want a second laugh on the reaction shot that comes next.
About this sound
The clip centres on the childish phrase my wee wee goes boing boing, delivered as a silly joke line with bouncy boing sounds underneath. At just over six seconds, it feels deliberately immature and playful, the kind of dumb humour that gets a laugh because nobody expects something so ridiculous to be said out loud.