They get that marriage isn’t “one size fits all”. They get that just because me and my husband are married, and my sister is married to her husband, and her grandmother and grandfather are married, the friend’s parents at the park are different and it is valid. Valid to them. Valid on a secular level. And she wil “get” that if one belongs to the Catholic Church it isn’t valid (or “OK” in kids terms).
Don’t be afraid to tell kids the truth.
I will give you an example of why this is important to be fully truthful. I was raised in a Catholic family with Catholic parents who sort of agreed with your suggested way of parenting. I came home one day when I was 5 (in 1960’s) and asked my mom what a mixed marriage was and why it was wrong. It was back in the day when racially mixed marriages were being challenged where I lived. It was still against the law for a black person to marry a white person.
My mom tells me a mixed marriage is when a Catholic marries someone who isn’t a Catholic. I accepted that and went about my business. When I got older, I can’t tell you the disgust I felt when I realized my mom didn’t talk with me about the issue of the day. Mixed race marriage was a hot topic at that time. She knew what I was asking and she gave me the Catholic answer, instead. Maybe she didn’t want to talk about race issues with me because she wasn’t comfortable with it. I don’t know the reason why. But by the time I was in middle school and knew better, it diminished my trust in asking her about the things in life kids should feel comfortable about asing their parents. She gave me a truthful (as far as Catholics are concerned) answer, but she didn’t give me the FULL answer, which would have been that there are all kinds of mixed marriages. She could have gone over the most prevalent ones. Instead, she zoned in on the kind she wanted me to know about, to the exclusion of the rest.
If you want your kids to trust you, you need to be in the habit of telling them the whole truth; not just the part you want to zone in on.