This is pretty long but I hope helpful. My family are in Indian but I was born here. My mother wears a sari all the time. I've always worn western clothes. I happen to have a disability and wouldn't be able to put on a sari easily. However even my cousins who where western clothes and sometimes a sari have to be dressed by someone else. It takes a lot of practice. Takes my mother a couple of minutes herself and she uses one safety pin at the shoulder. it takes her at least 15 mins to put one on me, even though I know I know exactly how to stand. She is doing it "mirror image" - both of us will have fabric over left shoulder but I'm facing her when she is dressing me - i don't have DC. but I suppose like when you dress your child and are doing up buttons the "wrong" way.
Wearing a sari takes quite a bit of practice. As others have said she needs a long plain skirt that is tightened and the sari tucked in. You need to remember to pick it up when going up stairs, if you stand on a sari it will come off or tear if you have a pin (yes it was very embarrassing 
). As with a long dress you need to learn how to go to the toilet!
Your daughter will need a generally tight fitting blouse as well. Take care when buying a sari as sometimes it comes with a blouse piece attached. The idea is that you cut that off and the tailor will make you a blouse the right size. In India all of these shops will have a tailor and quite a few in the UK. Buy a light sari, some have beads which look lovely but are heavy and take getting used to.
There are now ready made saris. They look like a sari but are already made up in a skirt. I've been looking to buy one. I've found them quite expensive - mostly they are aimed at brides who are marrying an Indian and want to wear a sari for the wedding family occasions.
Saris are beautiful. If your daughter really wants to wear a sari maybe see if there is anyone at her school who wears a sari regularly (teachers or parents) who might let her try it (including dressing her!). you'd be there and perhaps a couple of friends or they could have an assembly at school which my mum did a few times.
An alternative is to wear a Salwar Kameeez - long trousers and a long top over it (as with saris there are a few ways to spell that.)