The whole time...
The entire time I played this my reaction was, "Where the hell are the parents..?" I probably killed a few thousand kids; enough to deplete a large city of snotty noses... Not once did an adult run on trying to rush their kid off, or did the fuzz show up trying to ruin the party. I think it actually would have made it more interesting to progress from snot nosed children in their PJ's trying to whiz on the tree to the military trying to take revenge for the sheer thousands killed. If nothing else after about the 20th level I stopped noticing anything... different... I even figured out that if I get a machine gun (green present) and get lucky with a black present popping out before the last kid, that I can grab the rocket launcher just as the level finishes and the next level have nearly 200 ammo with rocket launcher bullets.
I suggest more content, more upgrades. Maybe make the tree retaliate with a hot wax upgrade, or how electrocuted cats to remind us of National Lampoon's Christmas Vacation. Maybe have a "special" kid walk on with a BB gun who shoots his eye out, just for laughs. You could possibly even do young villains like a 7 year old gremlin on his hover board throwing ornamental grenades. You could even have an assists upgrade where the elves might launch themselves from one end to another dropping explosive light bulbs or candy as lure for the children so they pack up. I'd also like to see one more thing... a reindeer with a bazooka... I don't care if the gun is in its mouth, or if it's standing on two legs going %u0CA0_%u0CA0. I would give a straight 5 and 10/10 just for that.