I'm late on this thread, but I had to chime in because I think I understand the OP's discomfort for both situations. Even when I was a Christian public praying made me uncomfortable. There is a verse in the NT about not making a show of prayer and I felt like public praying at restaurants etc. fell under this verse quite often.
For the second situation (the friend suddenly tuning out of conversation to pray silently), I do think it's slightly rude because the pray-er is removing herself from conversation without warning. It would be more polite for the pray-er to let her friend finish her sentence and say "Excuse me one moment", pray and then get on with it. When I was a believer and was out to eat with someone who I knew did not pray I always said my own silent prayer of thanks when I had a moment to myself or sometimes I would say my prayer afterward when I was alone.
For the first situation I think the rudeness was that OP's friends did not first ask if they even wanted to be led in prayer. If it was just one time, I'd let it roll off me, but if it was at every visit I'd say something. Guests in my home are welcome to pray or not pray, it doesn't matter to me, but I dislike someone trying to lead my in prayer without asking.
We sometimes pray before meals we sometimes don't. If someone is over we usually do (as there is generally an definite start to dinner as opposed to most nights which are kinda everyone grabbing what I made between showers. school nights suck.) The only people I ever invite over for a meal are people from my church so I know we share religious beliefs and prayers (Its not that I am opposed to other people those are just my closest friends). On the off chance that we have someone else over I usually ask them or one of the children, before the meal, if they would like to say the blessing tonight. i think that is a nice honor to give your guest. It also minimizes the risk of offending them with my theology (although we still cross ourselves before eating). And by asking before the meal it eliminates the whole presumption factor. I would have no problem skipping it if I thought it would in any way make my guest uncomfortable. for example, if I knew my guest was not Christian, I would choose to forgo the praying all together. I would not be comfortable with them offering up any prayers in my home as I would consider it idolatry. Actually i think it would be horribly rude for them to launch into a group prayer when I had made it clear by proceeding with the meal that it was not going to happen. Even if someone was part of a Christian sect I disagreed with I would really rather they not lead a prayer in my home. But at the same time I would not want to offend them and would not pray either. I would bless the food and offer thanks as I was cooking it, or before I offered it to guests. privately. There is no need to make a public display of praying before meals and prayer should never be used a tool to judge someone or make them uncomfortable. but yeah, if someone just launched into it would feel judged and offended.
When at someone else's house I would NEVER insist on prayer, presume to lead others in prayer or give any sort of "Aren't you going to pray message" which i what I got from the OP. There is no reason they could not have just prayed with the child, prayed before they come to the table or prayed quietly with him. Or hey, asked if they should pray with him or you were going to lead a group prayer. When someone presumes to take on the responsibility I see it as a judgment that I was not as on top of it as they were..
And I HATE the whole hand holding thing. Hate it.
And I never ever have a problem with anyone praying to anyone or anything in their own home. I am a guest and I can respect their traditions and customs. Nor would I mind if they skipped praying all together. I can take care of myself. And my children know that if we don't say prayers before a meal they should cross themselves or pray on their own quickly and quietly (I teach them that prayer should never be for a public display and so often it seems more about being seen praying than actual prayer and it has always made me uncomfortable.).
I am also mindful in restaurants not blow off my server while we pray. I think it is really rude to keep your poor server awkwardly standing there while someone goes on and on with the pre meal prayers.
As for the second situation with the woman praying silently. I can get why you are uncomfortable. She could easily A) wait for you to finish your sentence rather than just checking out. and B) say "excuse me for just a second". How hard is that? I often pray (in a public situation I would just cross myself or make the sign of the cross over my food.) before I eat but i would never just check out mid sentence. Although I think she is trying her best to not make a big deal out of it. I would just consider her awkward approach as a quirky part of her. I mean now you get it and understand it. Just roll with it.
ETA: Haha Anniegirl, I didn't read your post before I wrote mine. I could have just quoted you.
The truest answer to violence is love. The truest answer to death is life. The only prevention for violence is for the heart to have no violence within it. We cannot prevent evil through any system devised by mankind. But we can grapple with evil and defeat it, but only with love—real love.