This works best on canvas, as tape peels off easily - I've tried it with card and it is too easy to tear the card when removing the tape at the end.
So get some painters tape and mark out your design on the canvas. Curves can be tricky but it doesn't matter how much tape you use, just build it up until you have achieved the right look. Best to do this in advance of starting the craft as it can take a little time. Older children may like to try it for themselves, however.
Then get your child to paint over it. Last time we tried this F wanted to peel off the tape, this time he didn't attempt it once.
Let the paint dry, remove the tape and admire the awesome artwork!
This would be great to do with your child's name (which we did last time), or perhaps "I LOVE MOM" for Mother's Day? Let me know if you tried this out and what words you used.