When the Church concocted the 10th Earl's annulment, they explained away the child by saying he'd been forced by violence to consummate the marriage. Two children would have been a bit harder to explain away.
Some sources (like Tout in the old DNB) say that Richard and Isabel had one child, a daughter. This is a deliberate lie. The child was of course the son Edmund, raised to his teens as the heir, then suddenly bastardized and disinherited.
The whole annulment episode was pretty disgusting. But the descendants of Richard's 2nd marriage included the Dukes of Norfolk, the premier aristos of England. Best not to mention anything embarrassing. Also best not to supply ammunition to the lefties, they were revolting enough already.