The great thing about shapes in French is that most of them are very similar to English, so you won’t have to translate to your child. They should be able to figure the words out.
Here’s a fun little activity for any child, including the child who has no time to learn French. Make some instant pudding (any flavor will do. I think Chocolate or a dark color will work better than vanilla for this activity). Put some freezer paper in front of your child on the table and let them finger paint with the pudding. The way you can sneak a little French in, is by having them make different shapes. You can say the word in French and they can paint it. Then, you can have them repeat the word in French. Their reward for a good job can be licking their fingers!