How many out there have family members that they just don’t see on a regular basis? Even a yearly basis? Let’s face it, life and daily routine can get in the way of seeing those that we really want to see and those we really should see. Now imagine being a child who may have never met, or even seen, Uncle Bernard or Great Auntie Agnes. It would be natural for a child, especially a shy or passive child, to be apprehensive or hesitant in greeting family members that are new to them with a hug or a kiss. I imagine even more so if they are intimidating in some way, like being large in size or loud vocally. Heck, most of the time I don’t want to be near people I don’t see often or are comfortable around.
As a parent of an elementary school-aged child, I have raised my son
to trust his instinct on strangers. If it doesn’t make sense or feel right to him, stay away and don’t speak to them. I encourage that for him with family members that he doesn’t see on a regular basis. Just because we are related, this does not give a family member free rein to force affection on my child. If he is not allowed to touch a person that doesn’t want to be touched, why would I allow that to happen to him? If I tell him not to speak to someone that he isn’t familiar with, then why would I make him be affectionate with someone he doesn’t know? Luckily, my son is a social creature and it does not take him long to form an opinion of someone based on how my husband and I react to a person, as well as traits that we have taught him to look for in a person. I’m sure that our formula is not perfect, but it works for our 6-year old.
Is it rude? I do not think so. As I would never force a child to speak to me or hug me if they did not want to, I would never expect my child do the same. I do, however, expect adults to be patient and accepting of my child’s decision. I am not talking about a child who cannot pull themselves away from their tablet or PlayStation; I am referring to the child who is watching and paying attention to adults and guests, but cannot get over their apprehension and insecurity about a hug or a kiss for a ‘new’ family member. 
Don’t get me wrong, there are family members that we only get to see once or twice a year that my son is EXCITED to see, and there are those that are more local that he keeps his distance from. Whatever his decision is, to hug or not to hug, I defend it as his parent and expect other adults to respect it as well.