As Salaamu 'Alaikum Wa Rahmatullaahi Wa Barakaatuh,
Q: Is it permissible to write notes in the mushaf in order to teach the children?
A: Hani Ibn Beraik (hafithahullaah) said that there's no problem with this. However you should make the markings in the margins to show them their mistakes and to teach them the rules of Tajweed. Allaahu Aa'lam.
This q/a came via Live Salafi Duroos (Pal Talk).
Wa Sallallaahu Wa Sallam 'Alaa Nabiyyinaa Muhammad.