Potty training a toddler during covid with grandparents who won't tolerate underpants

Our grandparents are watching our nearly three year old who was getting really good at using the potty regularly. We sent him in underpants a few times and he pooped in his pants, and the grandparents got grossed out and went back to diapers or pull ups. Well, 3 months later, we have been using underpants at home and pull ups 5 days a week during the day, and I'm worried he's confused. He now refuses to use the potty at my parents and will use it maybe one or two times at my in laws. He still has some accidents on the weekends with us but I think he's ready for underpants full time, that his wet underpants and accidents will motivate him to stop having accidentally quickly, but we can't get any consistency. I've heard that using pull ups long term can be damaging since the kid will have accidents more often when making the switch to underpants full time.Should we lay off and just use diapers until he goes back to day care/school? via /r/Parenting https://ift.tt/2CiTrWu