Skroutz.eu

Very good choice for such an affordable bag, I did some "experiments" by buying 2-3 different types of cheap bags (under 5 euros) for a few A4-sized books/notebooks and this is the best one out of the ones I bought, both in terms of durability and "convenience" in putting in and taking out items.
In terms of volume, I would say it could fit A4-sized materials with a thickness that would reach two 200-250 page books.

My niece has loved it (exactly 1 year, it was for her birthday), although she hasn't found the right position to swing it more intensely on her own (parents mainly do it like a pedal).
The sound from it is bad (like a roaring dinosaur) but she doesn't mind at all, she actually likes it.
Good quality wood and soft fabric, it also has a belt (in some comments I had seen that some didn't have it, the one I got had it)

Easy handling, lightweight construction that doesn't seem high quality but at least in the photo it looks very similar to a device over 50 euros.
In terms of measurements, in normal temperatures that I have had the chance to measure so far (in more than 10 people in total and on different days), everything was within the range of 36.2 and 36.6, temperatures extremely satisfactory for me since I get such a small deviation with more traditional thermometers, only here I don't have to wait 5+5 minutes for two measurements.