It is hard to tell in your work situation if the person interrupting you actually needs to or if they are in fact simply exploiting your assistance. if they need your help, then you have to simply find the most workable compromise if that is possible, like asking them to write down the questions, drop you an email and them wait on a response. If they do not need to interrupt you at all, but merely are prioritizing their want to know over your work, then that should be addressed. I have had both types of situations. There are people that for some reason will bother you with minute things all day long if you permit it. I don't know if they just want to interact and are looking for reasons or so insecure they need constant reassurance, all I know is that it needs a boundary for there to be harmony.
I say something like this
It's not so much that you are a bother, it's that I struggle with
maintaining my train of thought when interrupted, especially so
frequently and it is impacting my productivity. Is there another way
we can handle your questions that interrupts my flow less frequently,
like perhaps an email or making a list so that we can go over several
things at a time.
As far as the kid portion, you state "one is too many" which sounds like you don't really like kids, or at least in your space. I would absolutely suggest you not have them over then. You can visit the parents at their own home or meet in public spaces. As a parent there is nothing I like less than visiting places that aren't child friendly. It stresses me out, it causes me to have to do a ton of correcting of my kids that they aren't loving (like asking them not to touch this or that, usually with total lack of anything they can do, so they are bored and antsy). It would be better for everyone then if you avoid that situation as it's seldom anything you have to do. There are usually very pleasant ways around it like meeting at a park or something else. It also means you can visit with the parents far better anyway, as they can focus more on talking to you versus peeling their little one off climbing your bookcase.
If it is some situation where I have a parents apologizing on behalf of their child, even if I do find it stressful or irritating, I simply say something like
Meh, they are kids. The good news is it only last 18 years.
And then laugh. Two year olds are wild, three year olds cry because they asked for crackers and you gave them crackers they refused, then you ate the crackers they wanted. They make no sense. They do all grow out of it though and we were all kids once ourselves and likely just as irrational and irritating.