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 serious question, if you really don't want Google hardware or software, maybe Android isn't the right choice? The whole OS is Google project.

How about an older, but still supported, Apple device?

I'm of the "no apple devices" Android-only crowd, but given your requirements, is something to consider