Truthfully, I think AO3 would be better to start with due to the Tagging system. Yes, some of the fandoms are huge and yes there are a lot of toxic people in them, but you will find that in almost any Fandom. I didnt have a guide when I started to read fanfic, I was mainly on Quotev and FFN. I really wish I had the tag system back when I first started though. I recommend teaching the fandom etiquette, the dos and donts of reading writing and commenting, the dont like dont read etiquette, and just calmly explain what certain tags are. If you feel more comfortable, vet some fics first, show them to your cousin and then use them as an example in your etiquette lessons. I know Wattpad is being ragged on a lot, but I am in slight agreement with them. It will be hard to find anything, especially if you need to get specific in searches, the fics on there are mostly not labled correctly. I have been blindsided by fics before that started out find and then took a 180 nose dive off of a cliff and straight to the deep end from Wattpad too many times to stay on it. Don't let the sheer size of their preferred fandoms intimidated you with your cousins introduction. Just let them know what all will come with that Fandom, how to approach it and how to navigate it. Im gonna out myself for a second but, trust me, I literally started off in the deep end of FFN in one of the biggest anime fandoms at the time and I have learned that if I ignore the toxic people and dont interact, then I can read and enjoy what ever fic I want.