well yes, it does sometimes depend on the children
but it also depends on what the reasons for sitting in are.
If it is just convenience for the parent to leave both rather than just one, I'd feel that I'm not a babysitting service and would have to find a polite way to put that across ( not easy ).
If it's essential for them to sit in they absolutely must not interrupt the lesson, and they may need to be told - nicely - that you can't teach effectively with interruptions and that the pupil learning won't get a really good lesson.
I have a fairly new family who have been doing a bit of sitting in, and I haven't made a big thing of it yet, but am gradually going to suggest that it's not ideal; the children are getting to know me now and will be less self-conscious without someone else there.
I tend to welcome a parent (and so if necessary a younger child too) in for the first couple of lessons as a confidence building measure - so they can see how I work and that I have a safe teaching area, that sort of thing.
Good luck - let us know how you get on!