Well done for knowing this & making sure your DD does too, as a sufferer of PA, who had to learn this stiff & fight tooth & nail to get my teen daughter treat even though that treatment git her out of a wheelchair & ive. Been involved with the charities raising awareness etc I get fed up of the number of vegans I've met & know who are adamant that they can get everything they need from a vegan diet, whilst complaining of symptoms of deficiency, it dies my head it when it's so easily avoided
Avoid the seaweeds mentioned below until B12 is well topped up, preferably don't eat often as they do not contain B12, but a B12 analogue, that mimics B12 so that the body recognises it as B12, but it isn't true B12 & won't stop a deficiency & can even block absorption of the proper stuff if you eat too much. Some are okay such as chlorella, but not the most common food ones.
Marmite isn't a reliable source for a vegan either, there are no vegan foods that are good enough. You DO need a good supplement as per vegan society advice & as above, a few others such as calcium too.
Injections are always best as you get the full dose whether you digest B12 well or not, which when not taking in large amounts in meat or fish etc, may be needed. You also need to take folate, iron & complete B high strength B Vitamins to get the best out of B12. Otherwise you just wee it out, as it only stays on your system for 48 hours & needs to be active to work.
This is the best one we've tried & was recommended to us by PAS & other B12 charities
A dissolve under the tongue supplement is best as it's a hard vitamin to absorb in the gut.
Comes in other flavours too...
Jarrow Methylcobalamin B12, 1000mcg, 100 Lemon Flavour Vegan Lozenges (1000mcg, 100 Lemon Flavour Vegan Lozenges) https://www.amazon.co.uk/dp/B002FJW3ZY/ref=cmswwrcpptai_hxaUDbQ4HPP80
Solgar High Strength Complete B vits contains full doses of the B vits
Including the folate she needs & a full dose of B6, which B12 will also use & deficiency can affect periods badly. Also recommended
Ferroglobin is good gentle on the stomach iron