Can anyone recommend a mobile phone with the following features?
* no Google services (e.g. by running an alternative rom)
* not too expensive (it's for children)
* not too bulky
* gets updates
* runs Signal
* at least 4G
* not produced by Google => no Pixel devices

EDIT: * no iPhone

I still own a Samsung A3 2017 which would fit those requirements. Unfortunately, I did not manage to flash it with an alternative rom.

#PurchaseRecommendation

@a32
You can disable Google services on the A3 using ADB (I can provide instructions if you like, but very easy – I am not a dev), at least sufficiently to make it "mostly" Google-free* I did this recently with an S7 and it is working fine with F-Droid & Aurora Store.
I was considering /e/OS, but wanted to keep the default camera app.
* – not totally so as still some minor background system services from Google like using their time server.