Depending on what 17.5 you are using right now, 13.5 may or may not make a big difference. When our SC Stock racers switched from 17.5 to 13.5, they did not feel much of a performance boost other than that the trucks were easier to jump.
10.5 and 9.5 motors are still fairly easy to drive and may be a better choice for a 'real' next step, unless you intend to buy one of the current (pro) stock motors intended for high-profile spec racing. Most probably you can get a better deal on one of the older 9.5/10.5 motors e.g. by Vampire Racing or LRP/Nosram. Ruddog distribution had a clearance sale going on, maybe similar bargains are currently in the UK.
Adding boost can make a huge difference, but also ruin your motor if you don't watch temperature. If your lads like to run a complete battery during practice and not quit after a 5-7 minute run, I'd say better safe than sorry and not increase esc timing too much. Using a normal non-blinky program with a 13.5 or 10.5/9.5 motor should be a good compromise. When I first let my son drive an 8.5 on an astroturf track, he was a bit intimidated at first, but from lap two he was grinning from ear to ear. As long as the grip is there, the kids will get used to more power faster than we old dudes.
With the 22, you've got a durable platform that will give your children confidence and they may catch up with you faster than you might like!
