AIBU to worry MIL may need a GP check for dementia?

No you should not leave the children with her.

My experience is that individual instances alone are not noteworthy, it’s only when you put them all together you get the bigger picture. I’d contact her GP

With my MIL it was Vascular Dementia to start with and that generally doesn't cause the memory issues that appear fairly early on with Alzheimer's.

She just started being odd, not able to cope with basic life admin, money, saying inappropriate opinions about e.g facial disfigurments when in cafes etc.

Have a look at Dementia UK website for the earlier signs for each type.

A diagnosis is useful for getting in to the 'Dementia system' within NHS, Social Work if needed etc but ultimately there isn't anything that can be done. There is a medication for Alzheimer's but that doesn't really make much difference beyond a few months.

If you haven’t already get POA set up.

What sort of things are you talking about? Wouid she be amenable to setting up GP appointment if you asked her to

If you are starting to have doubts, then don’t leave your DC with her. Things can change very quickly.

It was hard to tell with my mum as we only became more involved after my dad died, but he’d noticed (and not told us). She started having more trouble with numbers - adding up etc became difficult. He would insist she only use familiar banks and shops, I guess he may have had experiences where she’d become confused. For us we noticed her anxiety go through the roof, and she stopped being able to do internet banking, then her laptop, then her mobile phone. It’s not necessarily forgetfulness that comes first, but loss of executive function / working memory so it’s difficult to learn new things or retain information and focus on a task.

Get her checked - she may be at the cognitive impairment stage, and many people never go beyond that, but better to keep a watchful eye as stressful situations / events can make things get worse quite quickly. Wishing you luck.

My mum used to go on and on about my Dad " not being right" and " he's not the same" although there was nothing major.

I was living hundreds of miles away at the time and couldn't really judge it.

But her instinct turned out to be right.
He was diagnosed aged 70.
So If just say trust your gut if something feels off.