I hesitated to mention sand paper for a few reasons. Two of them are your son and daughter. I do not know their ages, but I, having raised 7 children, and are well aware of their acumen when it comes to things they are not sure of or have never had any familiarity with etc cetra.
The third reason is sandpaper itself. If not used properly it will very rapidly ruin the object it is being used on. Cue tips are, for all practicale reasons, a fairly delicate part of the cuestick. Using sandpaper on them is not a good idea unless one knows what he/she is doing.
As for shaping, you can put a nickel or a dime on a piece of paper and draw a half circle. Depending on the tip you are using you can use that as a template to shape the cue tip. If you and your family are beginners I would strongly suggest 13 mm. It is becoming the norm on many sticks.
After you are using the cuesticks for a while they will get dirty and possibile sticky from the small hands. RULE OF THUMB.....NO FOOD OR DRINK IN, ON, OR AROUND THE TABLE. Clean hands make for clean cuesticks.
As for cleaning the cuestick you can do it very simply. I use a Mr. Clean Magic Eraser that you can buy at your local market. You can use it dry or SLIGHTLY dampen it. After that wipe it down with a soft cloth and then GENTLY WIPE IT DOWN WITH SOME 1000 GRIT SANDPAPER. When it is smooth put a couple of coats of carbanuba wax on it. Same as you use on your car. Ther is no need to buy all the products you will see out there.
This is my method. Other people have different approaches. I believe in KIS. Keep it simple.