Magic Valley Cinema 13 in Twin Falls is now charging for infants to attend movies.

According to Magic Valley Cinema 13 policy:

2 and under are free to "G" and "PG" movies only, when sharing a seat with a ticket holder. All ages require a ticket to PG-13 and R rated movies.

No child discount for "R" rated films. Children of all ages will be charged full general admission for all rated R movies. No one under 17 is permitted to attend any R-rated movie unless accompanied by their parent or legal guardian over the age of 21. - Cinema West

Simply said, kids 2 and under are free to G and PG, if they sit in your lap. If they take up a seat, you need to pay for a child's ticket.

It's your choice to take your baby to PG-13 and R movies, but you'll have to buy them a seat regardless of where they sit.  And if you're taking them to an R movie, you pay full adult price for them.