Why is my butt sticky?

I questioned the kids as to what might be on the toilet seat.. no one knew.

After I finished making dinner I went to the bathroom and investigated. Toothpaste. It was the kids kind transparent pink on a dark green toilet seat, something you can not see.

My child (2) paints stuff with toothpaste, apparently now she paints toilet seats too, this is added to the list:

Walls
Bed
Carpet
Deck
Swingset
Dog
Grass
Outside of House
Herself