Waves to SueW, I am doing a 30 point maths course (MST121) and I also did the one SueW did (I'm guessing) last year (MU120). I am a SAHM, and both my children are at school. When I started my first maths course, dd2 was still doing pt nursery, and it was quite frantic. My day was drop dd1 at school, go home and play with dd2 and sort out the house, have lunch, drop dd2 to nursery, rush home, do some study, collect dd2, rush to school collect dd1.
Now, they are both at school full time it is so much easier. I would start slowly, and then see how you go. Studying with the OU can take a long time, it is much faster to go to a normal uni. However, you cannot beat the OU if you are a parent, because you do not have the childcare problems and expense.
You say your 3 year old starts nursery in April, but you have not mentioned whether s/he is doing full days or 2.5 hour sessions. If it is the latter, you will probably find that you are spending much of the day chasing around taking and collecting from nursery. 2.5 hour nursery sessions are a good taster for the child, but they are not for the convenience of the parent, so they are only a little help for the student parent.
My biggest bugbear with the OU is their over-reliance on February starts. If I want to do another maths course at level 2, there is only one start point, in February. This would mean that I would be studying during my children's 6 week summer break. For me, this is not an option. I want my children's summer holidays to be free for days out or visits to the park, not watching me study stuck indoors.