There are benefits to WFH - less time commuting, more time with the family, less expense etc. I know MN as a general rule frowns upon WFH but some people really do prefer it.
I do think that there are some things which are unreasonable - for example, expecting a SAHP and/or child to creep around their home and not make a noise. If you choose to WFH, then the noise is something that you have to suck up - or else go into the office.
But disruptions? No. I think it's entirely reasonable to expect the OP to tell her DS no, and ride out the consequences. Two-year-olds aren't renowned for their reasonable approach or logic, so he's bound to be a bit upset. But he will learn that when the door is locked/closed, Daddy is working and not available.
Have you considered putting a sign on the door? That could make it easier for DS to identify/learn that Daddy is working and therefore not available? Visual aids really do help children learn and I know a closed door is a visual sign but I think a proper pictorial aid might help your DS learn.
I'm trying to avoid using the B-word here OP because honestly, I've been there and I know it's hard. But sometimes you do need to tell your child "no" and they need to learn that they can't always get what they want.
What would you do if your DS saw your Tesco shopping being delivered and there was a big bag of sweets that he wanted to eat? If he kicked and screamed because he wanted the sweets, would you give them to him just to calm him down? Pretty sure you wouldn't - and this is the same principle.
I don't think it's reasonable to force the OP's DH to go to the office because OP isn't willing to teach her child that the answer is no. As your DS gets older, and especially when there's another baby in the house, that flexibility re working patterns might be bloody useful.
Another thing - when you say that Daddy isn't available, does your DS know what to expect? Because 2-year-olds tend to think in absolute terms - "daddy has gone forever!!!" Have you considered doing a visual diary for him? If your DS can see when his daddy will reappear, it will reduce his anxiety.
I promise you OP, I'm not trying to have a go. I've got SEN twins and I've been on both sides of the WFH coin - both the parent who doesn't want to be interrupted, and the one trying to keep the DC away from the one working! So I really do appreciate the difficulty for everyone. You should be free to use your home and garden how you want without having to keep the noise down so as long as your DH isn't trying to insist on that, I think it's fair enough for him to WFH.
**Obvs as I have SEN DC, some of the things I've suggested above come from "my world" of parenting DC who have communication difficulties. But many SEN techniques work well with neurotypical DC too, especially when they're very young. Happy to expend on anything if it would help!