Nanny - auto enrolment for pension whilst on Mat leave

When the pension scheme starts in October (if it starts, as it could be delayed by up to three months) then you would be assessed for eligibility as per any other member of staff. So to be auto enrolled you would have to have a monthly income that month of £833 or higher. During your first six weeks of maternity leave, you get 90% of salary, so you may well be eligible for being auto enrolled into the pension scheme.
After that initial six week period, your salary would drop to SMP, which at £148.98 would not then make you eligible for auto enrolment.

My employer yesterday suggested that I would be auto enrolled but no payments would be made yet because of SMP and so it'd only be minimal anyway and perhaps not worth it.

I would agree to a point. When on SMP you would not have to make contributions but you can choose to do so. You would meet the £490 monthly amount to have the right to opt in and get employer contribution. In the initial six weeks of maternity leave, when you get 90% of salary, you are likely to be eligible for auto enrolment or have the right to opt in (you have not said what your salary is, so I am guessing that it is probably £833 gross or above per month).

Your employer can delay the start date. That might be something they would look at so you were not automatically enrolled during your six weeks 90% pay period. It would depend on the exact dates involved.

Regardless, your employer has to have a scheme in place. If they are getting a maternity cover nanny, then they will need to carry out an assessment of that nanny's eligibility for auto enrolment.

Whilst you are on maternity leave, do you want to be contributing towards a pension scheme? The amount of money you get whilst on SMP is not very much, so the contribution will be tiny.