That was such a tragic event. In a way I could see why the Queen may have waited or was advised to wait to visit eight days post accident. The immediate aftermath and rescue/efforts are going on and to visit right away could hamper/deter those efforts.
I'm not saying HM wasn't aware of this, but the press, even in 1966 was apt to be critical if HM did show up immediately after the tragedy.
I remember watching the coverage when HM and Prince Philip visited the site and both were deeply affected.

I had hoped maybe The Earl of Snowdon or Lady Sarah Chatto would have accompanied the King, Queen, and Prince of Wales. Their father, with no fuss or fanfare, traveled to Aberfan immediately. He helped coordinate rescue efforts and did what he could to get more help ASAP, but his most important contribution was sitting with the families of the victims, listening to them, comforting them, and just being there for them. In many ways it was his shining moment. He even brought his own shovel. I hope his children honored the victims and their families in some way today.

https://www.townandcountrymag.com/society/tradition/a29280373/lord-snowdon-antony-armstrong-jones-aberfan-disaster-response/