Legoland is 38 miles from Newbury:
https://www.legoland.co.uk/
(There are two Premier Inns nearby in Slough)
Thorpe Park is 50 miles from Newbury:
https://www.thorpepark.com/
(There are several Premier Inns in the area)
Chessington World of Adventures is 60 miles from Newbury:
https://www.chessington.com/
(There's a Premier Inn in Chessington and several others nearby)
Didcot Railway Centre is 19 miles from Newbury and open this weekend for their Heritage Diesel Days:
https://didcotrailwaycentre.org.uk/product.php?xProd=29
(Premier Inn's 'Oxford South' hotel is actually located in Didcot)
Paultons Park is 43 miles from Newbury. (It would probably be my own choice to take a 7-year-old, as it's particularly aimed at younger children):
https://paultonspark.co.uk/
(The Southampton West Premier Inn is next door to it).