My daughter and niece wants to see a different movie from me their 9 and 12 would they have to be accompanied by an adult to see a PG rated movie like ralph breaks the internet.?

Nope! They can go alone. My friend drops her kids off at the movies all of the time. PG is really no different from G, it probably will have a little humor in it that only adults can understand which is why I love those movies myself.

 Your kids can go alone yet the rating suggests you might want to attend with them because there are scenes in that movie that might need an adult to explain to them... The official meaning of the PG rating: PG – Parental Guidance Suggested Some material may not be suitable for children. Parents urged to give "parental guidance". May contain some material parents might not like for their young children.

They can go along e, shouldn't be a problem.