OP, YADNBU.
I'm a teacher of 6 year olds and if one had an injury like that, I'd expect the mother to be phoned straight away and I'd certainly be out to see the mum at home time.
A blow to the face like this, plus a fall, can leave a child very shaken, as someone else said. I once had an 11 year old who tripped while skipping in the playground. She hit her face very hard and had a huge lump/bruise.
I took her in myself, her Gran was called as she was nearby. I bought her a tube of arnica to put on her bruise, obviously I gave it to her Granm who was very grateful for it and for my concern. She stayed off the next day as she was sore and shaken.
(I have quite often bought arnica cream for children who have had very bad bumps, sent it home in the Boots bag with a note to explain how effective it is for bruises. I speak from experience!)
I'd be furious if I wasn't informed, tbh. A blow to the face/head, should be closely monitored. We once had a boy taken to hosp with concussion after a fall.
I would go in and speak to the HT, but not all guns blazing! You can express your displeasure calmly and ask for an assurance that you will be informed if she has another bad fall like this.
(I would have said nothing as a child, and my Mum was the DHT! I did actually once have a bad fall, a boy slammed into me, I fell, battered my face on the ground, was stunned for a few mins. Someone took me in, no ice packs in those days. I'm sure I saw my mum and got a hug, but that was it. I had a terrible black eye which turned every colour of the rainbow.
I was 10 then. Another time, an iron drainpipe which had rusted, fell on my head!!! I am absolutely not kidding!!! I was 7 at the time. I remember a couple of older girls taking me up to the staffroom to see my Mum, but there was no incident report or anything. Hell, we didn't even have calpol at home or anything like that! I'm going back almost 40 years here!
I also once slammed the car door shut on my thumb, I can still remember how painful it was, and for days after. Still no calpol. Just didn't seem to exist. And no trip to A &E. We live in a different world now!
Sorry for digressing, just noting that we tend to medicate our children much more easily now, rightly or wrongly. That's not a criticism of anyone, I do it myself. If anything, I feel irked that I wasn't given any medicine for these accidents 'cos they weren't run of the mill! Could you imagine a pipe falling on a child's head these days???!!!)