There are no statistics and I doubt there ever will be . I suspect more dont return than do return.
JMO if a family member R’L has gone OTD, you should maintain a good relationship with them, but keep some house rules (Like no bringing in Pork into the house or using the smartphone on shabbos in front of everyone) most will be respectful of the house rules
If you R’L have a non-jewish grandchild, I have no advice for you