Ex husbands mother wants to see grandchildren?

im not sure on what to do as my ex husbands mum wants to see her grandchildren even though they have hardly seen them they roughly see them twice a year maybe 3 times a year this was even before the divorce, the kids aren't really themselves when around them i think its because they hardly see them. during the divorce there has been allot slagging off towards me sometimes this was in the presence of my children. my ex husbands no longer has anything to do with his mum due to a feud, now i no for a fact that if i don't accept the offer of my children seeing the grandparent then she will get solicitors involved, just dont know were i stand on this one and i dont want to put my children through any stress off seeing them as they dont want to see them but i dont want to drag it through court if i havent got a good case, also i may add that they are not blood grandparents as my husband is adopted by them. please help desperate for some advice.