How old are they? Have you seen this DIY nail polish dipping craft? Don’t know if I’d want it on a mug necessarily because I don’t like to have to hand wash dishes, but I was thinking about a vase. You could even use some old jars for the vase ... and a bouquet of flowers. Nail polish comes in some really great colors these days!
How about a bouquet of tissue flowers ... search an online video.
I've decoupaged small bowls to use as candy dishes or to hold potpourri.
You can grate color crayons onto waxed paper and press to melt the crayons then cut into shapes to hang in the window.
Hand paint flower pots and plant annuals (or give with a packet of seeds).
Theseare our favorite coasters and so easy to do. If the grandchildren aren't old enough to do the machine sewing, maybe they could put the pieces together and then turn them right side out after you do the sewing.
Will look forward to your report on what you do. Sounds like fun!