Hi ElinElin:
You sound a bit outnumbered. First of all I suppose the question is are you a single parent or do you have a partner there that can help support you?
Our solution to this problem was for one of us to do reading with DD1 whilst the other helped DD2 have her bath/ shower and then visa versa. In this way both DDs got a chance to read aloud with one of us and/or have us read with or sometimes to them.
Since you have a week to do the school homework - is this something that can be fit in at a different time. Perhaps at the weekend whilst your DS is napping? Or could your partner/ friend watch your DS and give you a clear hour to work with your DD?
Now keeping your toddler DS distracted for an extended span of time is always tricky. Does he like colouring? Could you get lots of crayons and paper & set him up on his high chair and tell him that he should do his 'homework' whilst you work with his sister on math?
Would something like mega blocks/ wooden blocks distract him? Would toy cars distract him? Or puzzles?
I know that there is a lot of press out there against letting under 3s watch tv and every family has their own opinion - but perhaps 1/2 hour of watching a show you approve of would give you the spare time to be working quietly with your DD elswhere (nearby in the kitchen, at the other end of the room, etc...).
It is nearly impossible to persuade a toddler that you should be paying attention to someone else/ something else - so I do feel for you - but this will pass. Try and build a routine (so regularly doing homework at certain times) so that it becomes so much a part of his normal week that he doesn't question that you're working with your DD. If you can persuade him that he's working on something too - a puzzle, a drawing, reading a book (possibly just chewing it) doesn't matter - but if he can be occupied and pretending to be a student too he may give you the space and time you need to support your DD.
Finally - can some of the maths work be done through gaming - whereby you get her on to the website and get her started, but she can pretty much do the work from there?
Useful websites:
BBC BITESIZE KS1: www.bbc.co.uk/bitesize/ks1/
Woodlands Junior School Maths zone: www.woodlands-junior.kent.sch.uk/maths/
Crickweb KS1 Numeracy: www.crickweb.co.uk/ks1numeracy.html
Coolmaths for kids: www.coolmath4kids.com/
Your school may also have a discount on on-line learning sites like Education City or Espresso - these are worth joining and again have lots of on-line support for learning, which your DD can probably work herself once you've got her logged in.
HTH