yes my children have and do see me naked or topless on a regular ish basis, we have 7 people in this house and one bathroom, it doesnt have a lock on the door, i shut the door and if the elder ones want to come in they knock, if i want privacy i tell them, if not they know i am naked and may choose to come in, if i am in the shower and one of them needs a wee etc they will come in and use the toilet, its the only one in the house!
if i am having a bath after i have got the 4 younger ones to bed and ds1 wants to have a chat about his day, something at school etc then yes he can come and talk to me if he wants, i dont force him, he will knock and say hello and chat, if i want peace and quiet and privacy i will tell him no.
yes my younger children who are 7, 4 and 17mths will sometimes get inot bed for a cuddle (well the youngest still co-sleeps) ds4 who is 4 will come in in the mornings for a cuddle if he is up early, he just snuggles up, sucks his thumb and often goes back to sleep.
ds3 will sometimes come in on a weekend morning, he may bring a book to read or just chat etc, its a nice relaxing way to start the day, until they all start climmbing jumping around etc!
and yes ds1 saw me give birth, i wasnt stark naked the whole time, i had yoga trousers and a vest top on for the early part actually, thankfully it only took 3 hrs (my other labours except the first one were also quick) i sat on the birth ball, walked round the room etc, ds1 sat in a chair most of the time, chatted to me inbetween contractions before things got going and talked to dp who as also there and the midwife who was impressed at his maturity and thought it was lovely he could be there.
BUT if he had wanted to leave at anytime the delivery floor has a waiting room for visitors he could have gone to, or he is old enough he could have gone for a walk to the canteen or even got a taxi the ten min journey home where my mum was looking after his 3 brothers. we also said if decided in labour i didnt want him there he would have to leave, or if there were any worries etc. we had talked it through in detail, he had seen vidoes of births, hosptial and home births etc and was pretty clued up about the whole thing.
when it really got going the last 1 and a half he sat quietly, it was actually a positive for me as inbetween contractions i would check he was ok and he was, i didnt scream and shout, i did moan and moo and had told him that i would do that, tbh he found the mooing a bit funny. and when i gave birth i wasnt flat on my back, legs akimbo i was kneeling up on the bed, so my legs werent that far apart at all, unless he had got down like the midwife did and leaned in to catch the baby he wouldnt have got more than a glimpse of anything,the baby as born, caught by the midwife and passed up between my legs to me to cuddle, like ds2 and ds3 had been (ds4 was a waterbirth so different) i turned around and had a sheet drapped over me then, i had kept my vest top on (which is actually unusual for me in labour but ihad a drip in and it would have been tricky to take the top off) ds1 then cut the cord and held his sister for the first time the look in his face of awe and amazement is one i will never forget, we have some lovely photos of that moment and actually its a memory he treasures. he felt honoured and proud to be there and to hold her (before dp did actually)
i dont konw what births you have seen what the hell just happened but they can actually be nice, they arent all traumatica nd full of screaming, nor do those at the birth have to see a vagina if they dont want to.
oh and re my children seeing me naked, yes they do that doesnt mean they see my vagina, i dont know about you but i have pubic hair (neatly trimmed but not too short 
) and unless i was to bend of legs spread etc or sit down with my legs spread what exactly do you think they are seeing?!
i agree with what others have said btw that 3 and 5 yr old should NOT have been in someone else bedroom, mine knew at 5 not to go looking in other peoples bedrooms when we visited and i would also have kept an eye on them to make sure they behaved, and they wouldnt at that age understand the sexual element of what htye were seeing, yes it would be a naked body but that doesnt equate to sex in a young childs mind.
you are making out like all nakedness is and has to be sexual well it doesnt, perhaps in your family but not in mine and in the many others i know of where actually yes children get in bed with their parents for a cudle and yes they may jump in the bath or shower with their parent or see them naked. there is nothing inherintly sexual in any of those things and i wouldnt expect a child to find any of it sexual either, if my 4 yr old sees my breasts they are something that his sister used to have milk from, he has no concept of them as sexual. basically waht mathanxiety said in her post atthe top of this page.