"My question is - is it "normal" or at least commonplace in seperation or divorces to have explicit provisions for sharing of childrens medical and dental expenses ?"
Maintenance is always reviewable so, yes, is one answer.
But if you are already paying "substantial maintenance" you could argue that that maintenance covers the above.
It really does depend on the circumstances - and your solicitor can advise based on your respective assets, liabilities, incomes and outgoings.
mf