Those saying they don’t kiss their own child on lips, are a bit extreme imhe.
theres strong links to weening and kissing to prepare food suitable for babies..jeez mammals do it with their youngsters! So, I think it’s natural instinct for parents and small kids and babies. Possibly grandmothers or aunts too doing mouth kissing as a natural instinct- but aunts these days will be mostly too well aware to think it’s ok.
but by 5-9 most kids will start to move away from taking comfort in mouth kissing but steer more to still having cuddles. Just like dropping breast feeding naturally. Again, all mammals take pleasure and reassurance in grooming, and in absence of body hair/fur cuddling is our grooming. Imhe, As adults we don’t touch in non sexual ways enough in some cases. Many older people who are isolated and lonely would massively benefit from human hugs for mental well-being. You can see this in good care homes where staff are still allowed to give short hugs. I’d very interested in studies between non sexual physical contact and depression 🧐
however, small child and male relative who’d not be involved in personal care like weening, nope. Not something I’d not question. GENERALLY. Smacks of lack of awareness, or something worse. BUT, it may also be because in a completely non harmful way , in 1950/60s when growing up his grandmother did this and he associates it with fond normal memerories of a close relationship. Just misplaced that he doesn’t get it might not be wanted here.
talk with him and grandma. Say daughter doesn’t want to, that you’re not keen…keep it light, don’t make a big deal unless he does. If he does, shut him down firmly…becuase if he argues it’s clearly not lack of self awareness!