Give 'em a hug? Then say "granny has a bad back so I would like a wave instead until I feel better" ? Then practice waving ...... one hand, two hands, waving whilst hopping, skipping ...... that's the kids of course ;-)
I hope that if grandparents have been in regular contact, by now any Virus will have jumped: what is it now, 4/5 weeks since panic began - also if people are in general keeping their distance and children aren't mixing, then the Virus will target someone closer?
These children have been together up until now so hopefully they won't be affected. Key workers are important - until it becomes known how likely it is for people to be affected, is there any other way?