There are too many variables here. Let them know what type of poly you used. They can take it from there.

And suggest no crayons or sharpies be used near it, if they will be unsupervised. A hand painted potential heirloom, painted with love by Grandma deserves to be treated with more respect than that.

I was one of those unsupervised children. You have to be ready! Today I am refinishing some of the furniture I ruined. Serves me right! :>D

The last time I took sharpie ink off of something it was with alcohol. It was on a finished wooden table top and it worked like a charm.

The last time I took a sharpie mark off of anything ( it was my white leather sofa!!) I used Ronson lighter fluid. Worked like a charm.

Rubbing alcohol will take off sharpie markers. It can be used on oil based polyurethane but NOT on water based.

Try Goof Off or Awesome

Rubbing alcohol or hairspray are my go to's.

I have used a dry erase marker to remove sharpie marks from dry erase boards and plastics. It might work on polyurethane too.

I have had some success removing very old sharpie marks from a polyurethaned wooden door by using a touch up marker over the black sharpie mark. The new marker ink is a pretty good solvent for the old. Worked better than pure alcohol.

No idea if it would work a polyed piece, but I've effectively used WD40 to remove crayon marks off walls.

I’m aware this is an older thread, but tips are always relevant. Try KrudKutter for cleaning walls before painting. Test first, but worked like an eraser for me on pebbled plastic.