Hi Dutchie,
As they get older they like the ride, well at least my eight year old daughter does. I metioned in my last post to Ameetw that I had been trying different pressures with my tyres and I think it may be worth considering lowering the tyre pressure carefully. I have found significant variations in the ride on standard sized 16 inch wheels by lowering the pressure just a few pounds. It may be the case that you may have to invest in a good electric tyre pump to up the pressure for lone high speed use.