The biggest thing I try and do with little ones is get them used to the water and board. If you have a pool it's a great place to start. If not I will work right off the beach or my swim platform.
First I get the board on them and show them how it floats. I emphasize to just relax and let the board float and their vest keep them afloat.
Once they get comfortable floating I give them the rope and show them how to hold it, to bring their knees up, keep their arms staraight, and talk about staying in that seated (just like if they were in a chair) position while being pulled. I tell them to try and keep their butts in the water for as long as possible.
Now I pull by hand. The small ones can practically get up by hand pulling. Once they are comfortable with that I intentionally pull them over to their bellies. This scares most kids, but as they get used to it I teach them to just relax if they can't flip the board over until the boat comes around, as some are just too small to flip it, and I teach them how to roll back over. They have to straighten out, get as long as possible, and really throw that elbow back to initiate the roll.
I do as few or as many steps they want to at a time. There's no pushing it. I don't want to intimidate them or push them too hard. The idea is to keep them interested. They all go at different speeds. Once they are ready we go for real. It can take all summer for them to actually want to try the boat, others are going in 1 day.
A 119 board should be perfect and last several years. I have some older pics of my two boys. The first one of my 4 year old on a 109, second one of my 9 year old on the 119. In fact 119 might be large depending on how big your 6y/o is.