Whenever Linda Brennan visits the Boston Children’s Museum with her daughter, she shows a card at the admissions desk and the two enter for just $4. Their afternoon of fun and exploration otherwise would have cost $34, prohibitive for the Waltham single mother.“If I didn’t have the EBT discount, I wouldn’t be able to take her to the museum,” said Brennan, who visits frequently with her 2-year-old daughter, Raine.