What do I do?

My daughter is 8 and a month ago I started dating a the father of one of her classmates and we decided until things were more serious we wouldn't tell our children or go public with our relationship.

Last weekend me and the guy went out for dinner and we were seen together by his daughter's aunt and long story short his daughter was told by her mother that I was his gf, no idea if there was any malicious intent.

Obviously his daughter started asking questions and I'm worried she's going to tell my daughter and I don't want her to get excited because she has thing about weddings after her dad got married recently.

What do I say to my daughter if she finds out about the relationship its going to be impossible to keep it secret from her now