Agreed !!

And I reckon 30 feet would be about right.

Also the non stretchy kite string would do.

The Cox models are not a good trainer for youngsters and are more likely to put them off the hobby I an affraid.
They are too heavy, underpowered and break.

I would recomend a strong Balsa trainer for learning on.
There are plenty of plans and kits and even rtf (ready to fly) choices, post back if you need pointers to some examples.

Good luck