I can help a bit with questions about younger boarders - I've worked in senior boarding but also ran a co-ed prep house for several years. Most of our kids were Year 6-8 so not too little, but we had a handful of Year 4-5 and one year we had a Year 3.
They mostly coped extremely well and they were all boarders for 'good' reasons (almost entirely military families). They had good relationships with their parents, who were mostly new to boarding and not necessarily at ease with the idea but who did very well to buy into it and be positive about it.
I remember saying at interview that I did not want the job if I wasn't allowed to touch the children, as the one thing you would hope every child gets at home is physical contact so if they want that in the boarding house, they should get it! Some of the younger ones would hold my hand walking to the dining hall or even sit on my lap if we were just hanging out. Others didn't need any physical comfort at all.
One thing that surprised me about younger boarders was that parents would send them without what I would consider pretty basic skills for boarding life, e.g. tying shoelaces, washing/plaiting their own hair. I expected to tie a lot of ties at the start of the year and to help with changing bedlinen til they got to grips with it, but I assumed parents would make sure their kids could deal with their own shoes! But it's fine, they help each other, staff help, and they learn.
I never really got upset comforting a child but the hardest calls to make were to a parent when their child was very sick or injured. Telling a mother who simply cannot be there that you are in hospital with their child is tough. I did get very upset once when a Year 8 student was having a full meltdown on the phone and it transpired her aunt had called her from her home country to try to get her to talk her father down from physically attacking her mother... that was horrendous. But not at all linked to boarding, except that the reason I was there was because she was a boarder. That's the only time I've ever cried with a student.
Generally I would agree with OP that if kids have good family relationships boarding will not damage that, and if they have distant relationships they will remain so. I think some kids benefit massively from the stability of boarding and others just enjoy it for the social aspect.
Full disclaimer - I was a boarder myself from 8-18 and I would consider boarding for my own children if they were interested. I think being on staff in a boarding house is a very special job and I absolutely loved it, I only stopped because it just wasn't doable at my particular school with children of my own.