While not all are (or maybe not all are), some are.
Live Science has an article that says:
n fact, finger-painting tots were quite prolific 13,000 years ago in the Cave of a Hundred Mammoths in France, according to Cambridge archaeologist Jess Cooney, who presented her findings last week at a conference on the archaeology of childhood at Cambridge University. The main art form was finger flutings, decorative lines made by people running their fingers along cave walls.
“So far, we haven’t found anywhere that adults fluted without children,” Cooney said in a statement. “Some of the children’s flutings are high up on the walls and on the ceilings, so they must have been held up to make them or have been sitting on someone’s shoulders.”