My DD started her periods in Christmas holidays. She too was 10, she was 11 last week. She's had 4 now.
I have been surprised at how well DD has coped tbh. After the initial shock she has really just gotten on with it. I did get her a treat that first month too - as I think it is something tough to deal with when so young, and I guess just wanted her to feel better about it all.
It was a surprise as I was a fair bit older at 13 but DD had been developing for well over a year, been wearing a bra for a year (also a B cup now) and has grown a lot (just over 5 foot) over the past few months. Her body shape changed a lot and she had shown other signs of puberty too. She hasn't reached the weight some say is the key weight, but all the other signs were/are there. DD has grown up a lot in the last year, well since Y6, and although still loves playing and being silly, has started to mature a lot too.
She gets some cramping and school have agreed to her being allowed Calpol Fast Melts to be kept in the office for her as and when she needs them. Still using Calpol and having period pain just seems wrong doesn't it? :(
I did tell school. I spoke to her teacher who takes them for many lessons including PHSE; she is also the mum to one of DD's classmates. They had done their talk about puberty and periods last year with her which helped. They have 1 or 2 toilets there with sanitary bins, plus know they can use the big visitors toilet if they want a little more privacy and a room with a sink, etc.
Coincidently 2 other girls from her class (of 11 girls) started over the Christmas holidays too, though they were a 6-8 months older and already 11. All in Y6.
I usually buy DD the teen ranges, although her first day is pretty heavy so we have some of the longer normal pads for then, to protect her clothes.
For school DD had a nice pretty make up bag - we got her a decent sized Jack Wills one in their sale as I knew it was something that would definitely make her feel happier about carrying it! In this she carries her pads, some wet wipes, some scented nappy type sacks and also a spare pair of pants. She also has another smaller make up bag which she keeps in her school sport's bag - again, just in case.
Last month it started the day before we went away and she was going to be swimming a lot. She was devasted but I supported her with trying a tampon. With some guidance she was able to use one for that day and although she didn't really like it, it did mean she was able to swim happily. She didn't need to the next days fortunately. I am not sure she would chose to use them again unless in a similar situation, but not really through choice.
DD does get a bit hormonal at the start each time; tearful for no known reason, etc. We just treat her a bit more with kid gloves at that time and make allowances for the day or so. It's hard still being a young girl at the same time as dealing with grown up hormones.
Give her lots of hugs :) and tell her she isn't on her own. It's a bit rubbish, but most of her friends will be the same too within the next year or so.