Your family is lucky to be around your kids at all. It's good the babysitting is out. I think there can still be a conversation though. Go over some positive character education that you got as a child that is not so much JW centric as it is humanity centric. You can give the JWs credit for that as a generosity though you know it's some stretchy b.s..
Tell your parents that indoctrinating your children with unique religious dogma is not ok, but if they want to include them in their Christian work of feeding the poor and assisting widows(cleaning, cooking, yard work) you would be happy for your kids to help shop or work at a food bank(as age appropriate) you would be happy to work with them in doing some practical Christian loving in your community. They can take that however they want, but it may give your family a new experience and something positive to do with the kids and you.