I cannot believe all the negativity about not wanting to help your own kids succeed. Don't put yourself down saying "but I am not a teacher" - surely we all taught our kids so much BEFORE the teachers got their chance, why stop? Maybe, just MAYBE, there is something you can explain better than the teacher, one-to-one, esp if they are an NQT/nervous - you have the opportunity to tackle it in a more round about way bringing in a hobby or interest of your childs, eg make it about buying footie stickers instead of apples and bananas, so they are focussed and FINALLY understand what the teacher has been trying to teach them - after all you have the advantage of not having 30 kids making noise/demanding your attention.
Many schools run maths courses for parents, "refreshers" if you like, so they ARE equipped to help their kids with the way it is done thesedays. If yours doesn't, ask them to! Ours also has a homework club which many parents like as it means 2 teachers get to supervise the homework and are there to explain everything.
FWIW most primary school teachers are ANTI homework (except reading, spelling and tables of course) BUT while it is compulsary lets SUPPORT the teachers AND OUR KIDS. One thing's for sure, if parents do not value their kids ongoing education, the kids will soon not give a toss themselves.
OP, sending homework back undone actually HELPS the teacher know who still needs more time on the topic or perhaps one-to-one support.
It is NOT about "if they can't teach them what they need to know in the time they have its tough" Better to let them know your kid is struggling than do it for them like some parents do!
Oh and finally, if you feel the issue is that the teacher is not bothering to set differentiated homework to suit the childs ability, complain, because they should.