Need advices to buy a Pen-tablet for my 8 year old girl

I have had an xp pen for a while and I am really pleased with them, for the price!
Some even work plugged to an android phone or tablet (The phone or tablet will act as the "computer" in that case.

https://www.xp-pen.com/product/1251.html
Or plugged into a computer obviously, and in that case she probably won't be needing a screen on the drawing tablet.

An Ipad, the cheapest, is a 10 in screen, so anything that size or a bit higher is good, I wouldn't recommend smaller, particularly for small, unsteady hands.
There are probably lots of good brands available (I know Huion is one, Wacom is still very expensive compared to those)

Things to take into consideration.

- Compatibility... You'll need a computer (or phone/tablet) to run those, so check that first

- Definition (how accurate the pen is to the screen)

- Pressure and tilt level. The more the better!

and obviously budget!

Apple & Google Play stores have plenty of free & premium drawing / painting apps.  For an 8 year old, I wouldn't be too lavish with the tech stuff.  Most 8 year olds have short attention spans and are apt to lose/misplace devices unless tethered to something.   You'll find used/refurbished iPads & Androids online for under $100.