Glass paints need a couple of days (at least) to cure before baking so if you are keen on doing this project I would start soon.
a big glass mug or stein
Pebeo glass paints, several colours (this has been my brand of choice for over 10 years, you can use whatever you like though)
Pebeo Outliner pen
Wipe down your glass with a swipe of rubbing alcohol to remove any greasy traces that would stop the paint from adhering properly
For young kiddos it's easiest to mask off an area for them to work on. Paint on the base or rim can wreck the glass when it fires
Have them use a small brush to decorate within the taped area, try to discourage them from painting too thick as it may run or bubble when baked
On the opposite side have them use the outliner pen to write Dad, the outliner pen is a tricky feat for most little fingers so I generally have them write Dad with Sharpie on a piece of paper...stick the paper inside the glass and trace their fabulously naive printing with the outline paint from the tube.
Let the project dry a day or so...carefully peel off the tape
Preheat your oven and bake the glass at approximately 325 degrees for about 20 minutes. Read the instructions on the paint to determine your perfect settings as all ovens vary.