I do very little for the same reasons you are finding. While it can be done, there is a lot of down time as kids wait or if you do it individually time the other kids are left semi-supervised unless you are crafting in the playroom instead of the kitchen. I do a lot of group activities and turn it into a game rather than a craft such as cutting out shapes and arranging them on a piece of board and then taking them off and playing again with everyone contributing to our "picture" but nothing is glued and nothing goes home. We put it away and play again another day.

Remind the parent that they are working one on one with their child and that is only possible when there is only one child in care not a group. Remind her that if you are working with another child to do their art it would be her child that would be left unsupervised/semi-supervised to entertain herself while you did that times each child so 10 minutes of attention out of the entire hour. As compared to what you offer and that is group activities where you all play together and interact socially and build up the positives of the group experience.

Once they get closer to age 2 they are more ready for the skills of art - cutting, gluing, drawing the world around them. For now you need to offer activities that will help them reach that point and that means going out into the world to see what it looks like (outdoor play), reading books (see what others have drawn), games that teach fine motor skills so their wrists and fingers will be ready for cutting when the time comes, and fine motor play teaches hand/eye co-ordination again needed for cutting. Remind the parent you are preparing them for art.