To recommend all sahms who might have another baby register as self employed...

[sillyoldfool]

...and pay their class 2 national insurance contributions?

If you register, and pay your NI you're entitled to the full amount of maternity allowance (£137 a week for 9 months) it doesn't matter how much, if anything, you earn. The NI contributions are less than £2 a week. If you pay them for 2 years you'll get the full amount back within a couple of weeks of MA.
So register, sell a couple of bits on test or similar, do a tax return once a year (very simple if you're not earning much) and then claim MA.

but HMRC already determines whether people are self employed or not.

so there are criteria they use.

if the criteria apply to you, you can pay class 2 contributions, otherwise you cannot.

Running commercially doesn't mean you can't make a loss, it means the business has to be viable. So if someone makes five items and sells them at a craft fair at £10 each, but the space at the fair costs £100 then that is not a commercially run business because it's impossible to make a profit. It's more of an income tax concept but is used for NI as well.

Years ago I told them I was working to try and earn money and was SE otherwise I'd be working and earning illegally!

If you have a small business you should be registered, even if that business doesn't yet turn a profit.

Yes, earning money is fine. Registering and not doing anything to try and earn money is not fine - I'm not sure what else we can do to explain the distinction.

to try and earn money

so it would have to be viable.

Which setting up a shop on somewhere like etsy almost always is , in theory, due to the v low running costs, which is what I suggested in the op.

The point I was trying to make, which I didn't realise until recently, is that your actual earnings are not what MA is judged on, it's the NI payments that matter. So the people (like me) who imagine getting MA when you're not employed must be a paperwork nightmare, shouldn't be put off, and it could be an incentive for SAHMs to set up an etsy shop or other small business, gardening, dog walking, whatever, and register as SE, knowing that even if their business doesn't make loads of money they'd still be able to get MA in the future.
Good for them, good for the economy that they're working and doing it legit not cash in hand!