Brainstorming for a birthday party activity

My son is turning 4 and we’re throwing a birthday party where there will be two other 4 year olds and one 2 year old. It’s in our backyard and I’m coming up with activities. I’m wanting to put up a sheet and let them have a free for all with paint. I was thinking about putting watery paint in water balloons, maybe some squirt guns. I was wondering if anyone had any other fun ideas on getting paint to canvas.