Dog growls at daughter when she tries to let him out of his cage

[Polyethyl]

Dog is almost 3. Daughter is 12.
This week dog suddenly started growling at daughter when she returns home and lets him out of his cage.
They've previously been absolutely fine and loving together and this started out of the blue. There was no incident to trigger this. Daughter hasn't interfered with or disrespected his territory inside the cage.
Dog is healthy. Loves his cage and goes into it voluntarily every night.
Dog is rarely in his cage during the day, as on the occasions my husband doesn't work from home he goes to doggy day care.
Daycare man says dog's behaviour is excellent.
The first time he growled at her after she'd opened the cage door, he stayed in the cage and she retreated out of the livingroom shutting the livingroom door and awaited my husband's return. Dog greeted my husband's return with joy. Two subsequent experiments where we've come home and the adult has stayed back and daughter entered house to open the cage have proved that this growling is not a one off.
Why would the dog suddenly decide that only adults may open his cage?

The naughty growling troublemaker

(Adult) dogs do not just suddenly change their behaviour to only one member of the family without there being some sort of trigger. Something has happened. If it was a change in behaviour in general - and not linked to one specific person - and he was reacting this way to everyone, I would say take him to a vet to check him out. But this is apparently only directed at your daughter, which says a lot, and over the course of only a week.

You can try taking him to the vet to rule out anything medical, but I'd be incredibly surprised if something hasn't happened to make your dog act this way.

That being said, no way should a dog that's 3 be in a crate for any length of time during the day - by that age they should be able to be trusted for extended periods of time in the house, even if it's one room, alone. So it could well just be he's sick of being locked up - just because a dog is happy to be in a crate at night, doesn't mean it's happy being in one during the day - for any length of time.

What's he like with your daughter the rest of the time? Is their overall relationship still good and only the cage opening an issue?

Yes. They play happily together. He comes to her call on walks. They snuggle together.
I have asked her if there's been an incident. She says not and I believe her. It would be quite out of character for her to do anything unkind to a pet.
Without question, his favourite human is my husband. But he loves us too.

He did, once, have a panic attack when I left him alone to sing at a funeral. Several months ago I went out for 2 hours and didn't bother put him in the crate. The neighbours had to use their key to calm him. We normally put him in the cage if we are popping out to ensure he doesn't eat the cat's food. So we assumed the panic attack was because I had deviated from the norm by not bothering to move him from his day bed to his crate. He hasn't had a panic attack since, so we thought the cage is his safe space when he's home alone. But now this....

We are only ever talking an occasional 2 or 3 hours in a cage during the day. Anything more than that and we call the doggy daycare man. Today I left him home because the weather is so hot it didn't seem kind to make him shlep round the shops.

It does sound like something happened when your daughter let the dog out of the crate to trigger this reaction, since everything else is normal.
I am not in anyway blaming your daughter or saying she hurt him. But dogs think differently to us and connect dots in different ways. Maybe the dog hurt themselves on the cage as they came out - mine often used to rush out and head butt the door before it was fully open. Or they were startled by a noise as your daughter opened the cage. It could be a multitude of things.
Is he alright with your daughter being around the cage when he is locked in? Could she throw treats in through the bars without opening the door to try to reestablish her being near his cage as a positive thing?

I was thinking similar. Could your DD have accidentally caught his fur, or tip of his tail in the hinge as she opened the door to the crate and now he's made that association?

What usually happens after she let's him out of his cage? I'd think maybe he doesn't want to do that thing he does with her and so thinks resisting coming out when she asks will avoid the thing he doesn't want to do.

Something has happened when your DD has let him out of the crate - that doesn’t mean she’s at fault, but more that a noise or something has coincided with her opening the crate and he’s made the wrong association.