Don't underestimate what your 6 and 8 year old can do. On one occasion a group of mixed age kids on bikes chose to try to race me on a slight incline. On my e-bike towing a trailer I was running at exactly 15 mph when the smallest kid of around 8 years old on a tiny wheeled bike very gradually eased past me. With grim-faced determination, his legs were whirling at an impossible rate on that low geared bike but he was clearly still able to input at least 200 watts to do that and probably more, beating all the older kids with him.
It was a council estate I was riding through on my way to the council tip, so maybe they rear tougher kids there.
.