Why would this man ask my daughter to move out of his way?

In what world is an adult asking a child to excuse me in a supermarket at all rude? Jesus.

I often experienced similar when our kids were younger.
Worst was an older man literally reaching over the head of my son to get to the coffee machine then trying to lift the hot coffee right over his head.

I gave him what for. Thankfully, the woman on the till gave him short shrift, too. He couldn’t see what he’d done wrong.

As they became older, they would ask if they could go to the kiosk at theme parks, etc. to choose their own food. Adults would often barge in front of them in the queue,

Again, I wouldn’t tolerate it and intervened. Tbf, most were a bit sheepish and apologised.

Not a 10 year old no, I wouldn't have thought she was making supermarket purchasing decisions either and also would have asked her to move.

Sorry op I disagree with how you went about this, you made something into a big deal when it wasn’t, your daughter was probably surprised/ shocked but then you added to it. People should move out of the way for others if they’ve said excuse me!

He probably assumed she wasn’t shopping, just waiting for you. So long as he was polite then I can’t actually see an issue…

I can spot a yellow sticker in M&S before I even get in the shop!

Why? Because many, many men see women and children as inferior and disrespect us in situations where they'd respect / listen to an adult man.

If you've not noticed this yet, be thankful, but please don't dismiss those who have.

You’re thinking far too much into this.

If there was a child stood in front of a fridge/food section I needed to get to and they were In my way yes I would say excuse me.

I agree with this^
He didn’t consider her as a genuine customer due to her age which was a reasonable mistake imo.
However not changing behaviour after OP’s clarification was rude.

Why ever not?
I, as a woman, ask regardless of age or sex, but then to quote MN I wear big girl pants and use my words.

Probably? Why wouldn't he? If someone's in your way, you say excuse me.

Was she plonked right in the middle? I cannot imagine the fridge being so narrow that only one person could stand in front of it and block it all. Maybe he rudely was telling her to move over so they both could stand in front of the fridge.

Things get competitive around the reduced section.Its an interesting learning and discussion point for your daughter.
Perhaps he was looking through necessity, either way it's rude.

He didn't ask the OPs DD to move out of the way - he simply said excuse me.

I have done that numerous times in the supermarket when someone is stood daydreaming in front of a shelf I need too get to. There is usually room for at least 2 people to look/access an area if they don't plonk themselves right in the middle.
Some people seem completely oblivious to the fact the others are also trying to shop at the same time as them.

Would he have asked another man to move out of his way? I doubt it.

"But small devil's advocate - if I came across a ten year old in the supermarket standing in front of a shelf I wanted to look at, I would quite possibly think that they were just loitering there while their adult shopped, daydreaming/bored etc, rather than actively perusing the shelves for something to buy. Unless it was the snacks sweets or toys aisle! So if I felt like they may be hanging around there for a while not actively shopping, and I wanted to quickly nip in and get something, I may well say excuse me so I can just quickly reach past to get XYZ and move on."

I would assume this ☝

I think i would have said excuse me to her as well. Ten year olds aren't usually the ones in charge of or doing the shop, I also would have assumed she was idly looking, curious, bored etc. And I always know exactly what I want and would be in and out of the fridge in seconds. (And can skim the reduced section pretty quickly!)

When you explained that she was looking for something he was wrong to ignore you, though.

Was there not enough room for them both though? If there were other people there, and there was no room at all, then yes he should have waited. But it sounds as if there was just your Daughter there. So what was wrong with him asking her to make space for him to look as well?

He was probably being a bit of a misogynist/ageist jerk and thinks a 10 year old girl is of little consequence compared to him so she should give him priority and space.

But small devil's advocate - if I came across a ten year old in the supermarket standing in front of a shelf I wanted to look at, I would quite possibly think that they were just loitering there while their adult shopped, daydreaming/bored etc, rather than actively perusing the shelves for something to buy. Unless it was the snacks sweets or toys aisle! So if I felt like they may be hanging around there for a while not actively shopping, and I wanted to quickly nip in and get something, I may well say excuse me so I can just quickly reach past to get XYZ and move on.

Why ask her to move? Because he knew what he wanted and was in a hurry? Why on earth shouldn't your daughter move?