School work is never "finished", so you have to set your own rules for the amount of time you are prepared to put to your school work. A little bit of this is about time management, and that's generally easier (I think) if you have your 'rules'........ so you work on Saturday mornings, but from 1pm, that's it - you are available to them. They know to leave you in peace and then it's time for them.
What time you do things also depends on how your body clock works / how well you can concentrate later in the evening / if you have a partner who is home same times as you, etc.
Does depend on their age though, and if you have a partner who is available to keep them out of your 'space'.
As they get older, then they can understand more that - if they do X, Y, and Z, whilst you are doing your marking / planning then you'll all be ready to go out sooner, whereas if they keep interupting you, or , if once you've done school work you then have to start on essential housework, you won't get to do the fun thing until later.
I've worked at different times over the years - we had a spell where I used to arrive at school just after 7am (dh got the dc up and dressed and took them to breakfast club) - which I actually loved. School was quiet at that time of day, I'd get a good hour and a half done before interruptions got too much and I could then fully justify saying that I'm not going to work this evening, sometimes.
If they are a bit older then doing things like lift shares to activities, so you only do half - or staying where they are and fitting in 50mins of work while they do their activity.
If they are smaller and just want to play in the garden, then set up and do some work out there if they just want you near.