My daughter’s also capable of eating on her own in school, but she’s a very slow eater (sometimes taking 1.5-2hours if no one helps her). At home, I let her feed herself, and then if she’s too slow, I feed her the rest to speed things up. When we went over to my in laws’ place, my MIL would want to feed her the entire meal, cut everything into baby sized pieces even though I said many times that there was no need to. I usually just said it nicely, and if there was any thing to reinforce, I let my husband be the one to tell his mum.
Thinking about it, though, it’s their way of bonding with their grandchild/grandparent, and all grandparents definitely want to shower love upon the little ones, so... I try not to get too upset about things not being the way I want it to be. Since my daughter actually has the skills to self feed, I’m not worried about her not being able to do so if grandma feeds her. I just encourage her to feed herself, especially at home, and then I’ve actually seen her tell her grandparents later on that she wants to feed herself. So.. I didn’t show any disrespect, and my daughter became more independent even at my in laws’ place.