AIBU to expect the host to keep her own bedroom?

I’m inclined to say Yanbu and that the host should give up her room. Bed size is kinda irrelevant, unless the kids are sleeping in there with her. What matters is room size and whether the additional space is needed to accommodate the two children.
This post is kinda giving off first world problem vibes (who has a ‘second home’ and then complains about giving up their room for a few days!?) if you are the host although I’m going to assume you’re not as you’ve assigned ‘Yanbu’ to the host giving up her room!

Friend A has right to any Ron of their choice. Friend B should bring or purchase air bed or similar of their choosing.

If Friend A offers to swap that is their call and very much above the required line. But why would they if bf is coming?

Yes I agree … kids should stay with family or their dad, and the friend gets the spare room

So are you the second home owner or the mum with 2 kids? What do you want to happen?

I don’t give up my bed for anyone. No problem for friend with 2 small children to sleep in a double bed.
Who are you OP?

I think giving up your bed in your primary residence is odd, unless you have a guest who can't sleep comfortably in the guest bed. For a second home, I feel slightly less strongly about this, but ultimately, I'd rather sleep in my own familiar bed. Guests usually just sleep wherever there's room for them. I'd expect the kids could sleep on a sofa or something else (blow-up mattress). (Most kids can sleep practically anywhere without discomfort.)

It would be very kind of the host to give up her own room so her guests can be more comfortable. But it certainly isn't expected or required. All in all it is rather inconvenient to swap, as the host presumably has personal things in her own bedroom which she would need to move, and may mean changing the bedding extra times.

My sister & her DH gave up their larger room to make my (larger) visiting family more comfortable. I was extremely grateful. When they visited us years later, my DH and I gave up our bedroom to make them more comfortable because of their medical needs. They were extremely grateful. It was somewhat inconvenient in both cases, but it was a nice thing to do.

I don't think I would have put myself out for anyone I didn't love. Fortunately we are quite fond of each other.

I wouldn't give up my bedroom for anyone. Would be awkward with me needing to get in there to access clothes & other belongings while the visitors were there.

nailed it

I wouldn't dream of giving up my bedroom for a guest unless there was a real necessity.

At 2 & 5, they can all fit in the bed. Both ages can still wet the bed etc and are curious enough to open drawers etc. They get the spare room. I'm overweight and can sleep in a single with my nine year old granddaughter.

Yes I am the same. Would give up my bed for my parents or mum to go on sofa bed. Anyone my age or younger had the sofa bed and/or blow up.

or can stay in my room if we are away (sister only really) and bung sheets in wash when they leave and remake bed!

Neither.
New boyfriend doesn't come over while friend A is hosting her friend and small children.

it's not unreasonable for friend A to keep her larger bedroom but it would be courteous to offer it to friend B, that's what I would do for a guest.

I’ll (well we’ll) give up our (super king, accessible on both sides) bed if it’s necessary for our guests - either elderly parents or couples with young kids who’d struggle with double that is only accessible on one side.
I would probably assume friend b would be comfy enough in the double room for her stay.

It’s been a LONG time since I’ve had a new bf, but I wouldn’t have bf stay over if it’s just a short trip for my friend; I’d stick to 1-2 date nights out/at his.

but each to their own and it’s the homeowners decision to offer or not.

I often give my bed up for guests, but no-one has ever given their bed up for me. When I move next month, I won’t be giving my room up again.

A standard double is fine for one person to share with 2kids 5& under. Would I want to do it every night of the year no, could I if needed, yes. For a holiday? No drama.

Expectingn a friend to give up their bigger bed in their own room is ridiculously entitled. Though some may offer it, it certainly shouldn't be expected.

Are you friend B? If so, its friend A's decision.

Are you friend A? It's still friend A's decision.

If you're neither, its none of your business and still friend A's decision.

Why has friend A invited her new boyfriend to stay when she already has multiple guests?

I think that is a bit of a stupid decision tbh.

Friend B takes the room that is offered, if she doesn’t like it or thinks it’s not comfy enough then she looks for a local hotel

(I think OP wanted us to pick up on the 'new' bit of boyfriend. I was obliging. I still maintain, new boyfriend = shag-fest. However, that is a good idea - kids in with Friend A = death to shag-fest.)

Host keeps own room. I would probably put a fold out bed or extra mattress in the spare room though.

This is a bit of a stretch. What’s wrong with the children being introduced to A’s new partner. Not every man out there is the child snatcher from Chitty Chitty Bang Bang.

Friend A stays in her own room. If friend B isn’t happy with her free accommodation she can pay for a hotel. It’s one adult and two small children in a double bed it’s not as if she has a partner with her too.

Friend A keeps her bedroom.
But then I don't give up my bedroom for anybody