Hi.
Any Birds and their offspring, of different ages, don't do well in being
'together,' unless there is some sort of barrier between them !

Tip(s)
I Would put the youngest ducklings in the Dog Crate, with their Mother. And ensure you wrap some sort of fine mesh, or netting around it, so ducklings don't stick their necks out and get injured by others.
I use fine mesh used to 'shade' greenhouses. I get it here in the UK, at a garden centre. It comes in a long roll, you cut the amount you need to go around "all sides" of the cage, and a bit of more as an overlap over the door !
I make a feeder and waterer using Milk Cartons that are cut so their is a long tall narrow slot, so Mum and babies can get their beaks in, but not any more than that. Otherwise Mum mucks the water up, and babies don't drown (!)

Tip !
With the Older ducklings, and their Mum, let them circulate around the others, in the cage, so they can "get socialised" without direct contact.
You can eventually bend some fine mesh wire or plastic type mesh into a
'curved pen,' so Mum in crate has a mini exercise run, that the others can go around ... but cannot touch those smaller babies that are inside !
Also...
I would try to make another secure home for Mum with older duckies.
Maybe a tall container, on its side, an ex dog kennel etc. Then circle this, and the crate of others, so all are held in the same place, but protected by external factors.

Suggestion :
Can you get a big roll of heavy duty galvanised wire, or panels of wire, and Zip Tie one bit off the other?
And remember to anchor it all down securely !!! So yours (emoji of ducks) are safe.
My Chickens and Ducks...
Live in pens made using "1 metre high wire on a continuous roll," don't cut it !
And I use UK 'garden digging forks' as 'spikes and associated stakes,' to zip tie to, at intervals along the wire roll !!!
It means, I can wiggle out the forks, move the continuous wire roll, to make a bigger circle, or side by side shaped rectangles, to be the size, or on different fresh ground as needed.
It works brilliantly ! Mine have been my permanent flock pens for nearly two years now.
* Seen those in the US, use these electric fences, that run off Solar power, with a storage Battery unit.
I couldn't find them myself, so came up with my wire roll and garden fork system ! And I use heavy duty black fruit netting, stretched across and anchored to each fork handle with a zip tie. This stops the flighty fowl inhabitants getting out, and from garden wildlife getting it.(*)
(*) If you have larger predators, maybe use roll of wire, across the top, for stronger protection.
It's all about safety of one from another. Duck to duck, duck to predator.
Mums can get very protective, in protecting their young, and can injury their own, by accident !!!
Think of every 'eventuality,' and you should be fine...
Hope this helps.
From me and Daisy duckling !
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