Hi,
I helped build the trails over Sandwell Country Park so hopefully I can help with your question.
My Daughters aged 4 & 7 both ride around the trails blue lines with the older child riding a few of the selected red options. So depending on your children’s ability and confidence levels should fine something they can ride.
It’s never too busy around there so you should be fine, though some of the hills are actually quite hard for little legs and little wheels so be prepared for a lot of pushing.
If that fails, there is plenty of other quieter trails around the valley that i’m sure they will enjoy.
Edit, There is also a pump track similar to the above video in Dartmouth park which you can cycle over to from the MTB trails.