I don't have a daughter, so maybe I'm missing something here, but I must admit I'm shocked by the very idea of letting children take norethisterone for non-medical purposes. It's a synthetic hormone, and like any other drug or medicine, it carries risks. If you need it for medical reasons, those generally outweigh the risks; but if you just want it for convenience on holiday, that's very different.
Its possible side effects include:
Headache (including migraine headache)
Feeling sick
Being sick
Stomach ache
Swollen or sore breasts
Change in weight
Changes in sex drive
Patches of darkened skin
Itching
Acne
Rash
Change in appetite
Feeling dizzy
Feeling tired
Feeling depressed
Breakthrough bleeding or spotting from the vagina
Irregular menstrual periods
High sugar levels in the blood
Difficulty sleeping
... AND MORE :(
Link to Boots Chemist drug info
Its long-term effects aren't really known, because it hasn't been around very long, but there is a possible increased risk of breast cancer in later life.
This student discussion board also has a lot of young women discussing how it didn't work, and/or made their periods heavier and more painful.
If your girls really don't want to use tampons or a mooncup, then I agree with isthistheend that cloth pads are a much better option than these chemical hormones.
I'm also saddened that there are girls and young women who feel uncomfortable about touching themselves and putting a tampon or finger in their vagina. At the risk of sounding like a yoghurt-weaving 80s hippy, does anyone else remember reading the bit in Our Bodies, Ourselves which encouraged women/girls to explore themselves, using a mirror as well as fingers?! OBOS has a website now. It's a bit clunky, but it has a self-guided 'tour' for women to explore their anatomy 
Women and girls who are comfortable and confident with their bodies are safer, as well as having more fun! :)