Yogurt should help her--acidophilis and lysene together would clear them up, but I can't make any recommendations as to the dose. What about speaking with your dentist? Her resistance must be low for her to keep getting them. Good luck.
ill give you a link to a homeopathic site if you d like def worth trying heres link
follow the instructions and put her symptoms in as best you can hemeopathic medicine can wwork wonders itd really worth trying
if that doesnt work id see a nutritionist in case its a small deficiency in something or give her a good childrens food suppliment
I used to get loads of ulcers as a kid. Everyone used to tell me it was a lack of vit B but I didn't believe it (and still don't today!) - I have a very good diet and even when I took vits with loads of vit B in them, it made absolutely no difference. I once had one so big that the doctor burned it out of my mouth with some silver nitrate (this was years ago - don't think they do this anymore!!)
Anyway, have you tried anbesol? It's fantastic stuff - numbs the mouth and I really think the less you feel the ulcers, the quicker they go away. I think when you know they are there, your toungue constantly goes over them etc. etc. and they burn when you eat and in my opinion, they stay around for longer. So get some numbing stuff and rub it on. (Check with pharmacy if she can use adult strength otherwise get the junior one)
Ime, the absolutely best thing for mouth ulcers is Adcortyl in Orabase. It puts a protective layer over the ulcer and contains something that promotes rapid healing. My dr prescribed it when I had an ulcerated cut in my mouth after I got knocked over(!) but you can buy it over the counter in a chemist's. It's about 5gbp. Hth.