You'd be better off going down the childminder route in this instance. Nursery will lock you into paying for entire afternoons which gets pricey. They don't usually charge an hourly rate (not where I live anyway).
Just for reference, my Mum was a nanny well into her 60s. For her, as an older person nearing retirement but still fit and well this would have been the perfect job for her - just doing a couple of hours, a few times a week.
She was trust worthy, qualified, and OK- she wasn't running around play grounds with toddlers but she'd get out the paint sets, make lovely home-made dinners, read stories, take them out for a walk around the park - depends on the families wishes really. She was basically a hired Grandma, haha. The kids she minded are mostly all grown up now. They still come by her house to say hello. She's Facebook friends with them all. But it was families and situations like you're in that were the jobs my Mum took on. She wanted an income but not full time work.
So if it's just 2-4pm - I think you could find something similar.
It's a shame you're Mum can't/won't help but not much you can do really.
Perhaps it's for the best though, you can do things your way, it might starve off some of the motherly 'unsolicited' advice! Haha.
I don't think Yabu or Yanbu - I think you're just disappointed and now slightly panicking but you'll find a solution and it may work out for you better in the long run anyway.