Welcom aBoard Richard.

I can't think of a word in English either.

In Québec French "children" (enfants) refers to your immediate progeny no matter how old they are, i.e. "these are my children Ginette and Diane" and "small children" (petits-enfants) refers to grandkids no matter how old they are, i.e. "these are my small children Patricia and J.P."

You also have "rear small children" (arrières petits-enfants) which means great-grandchildren.