Any reason my 7 year old can't take an antacid?

NO REASON. Find out reason why 7 year old needs antacid if no underlying serious problem occasional antacid use is ok.
Reflux. 7 years old can take antiacid for sure. But you need to make sure that it could be a signs of avid reflux and possibly due to h. Pylori bacteria that needs to be properly treated.