You're so awesome for doing this. My mom wasn't an artist, but she would write me a note every morning and they meant so much to me. Your kids are so lucky to have you as a dad. Doesn't hurt that all your pictures are incredibly cool and creative!
I do a set every day. each bag takes from 10-30 minutes depending on a multitude of factors (not the least of which is whether I had too little sleep). Sometimes I do them at night, sometimes as I brew my morning coffee.