Taylor rides at 22.5 - 23.5 mph depending on line length. She uses a rope length of 65' when she is doing a competition run which includes everything 2 wake -180's, switch 180's , grabs, etc, then she has just started doing TS front rolls. When she plays she rides at 80' @ 23.5 mph going w2w both directions. She won't do any 180's at this length yet.
The way we got Taylor out was on skis. She started out at about 3 1/2 years old. Then moved to wakeboarding at age 5. You have to keep it FUN for them or they will be scared. What ability is you kid? Meaning does he like water? Does he like riding in a tube? Make it as comfortable as you can for them. Go whatever speed they feel comfortable with. Maybe get out on a line and ride beside them. If he /she has friends that do it, let him watch them. Seeing someone else do it and LIVE makes it much easier for them to decide they can also do it. Below is a link to my daughter riding. Hope this helps. Most important keep it fun for them.
http://www.wakepics.com/view_single.php/360-Picnic-Table_wakeboarding-photos.html?medid=18468