My oldest is now 10, and every year, one of the things we o for Father's Day is make a t-shirt for him. Print several pictures of your child onto transfer paper and iron them on the t-shirt. Add hand prints with fabric paint (we learned to spray the hands with just a touch of cooking spray before applying paint - it helps with cleaning their hands easily and doesn't effect the print or t-shirt). Then I use the paint comes in a bottle with a fine tip and add a cute saying "Best daddy...", "We love daddy..."
With one child, they can do two handprints. If you have more children, each does one print.
I always try to make sure that one of the photos I use has a pic of daddy with the kids.
NOTE: a large piece of paper (newsprint, wrapping paper, grocery bag, etc.) between the front and back of the t-shirt while doing paint, so it doesn't bleed through to the back - lesson learned years ago :)
Good luck!