Cute little crushes and tiny first kisses

Last week we had our friends the Anderson's over for pizza. They have 2 boys, Kaden age 6 and Kyle age 4. The kids were playing in their room for most of the evening. We noticed that Kaden and Rylie were being quite friendly with each other and flirting, even thought Rylie has no idea what flirting is, we could tell they had little crushes on each other. A few days later we find out from the younger brother Kyle that that night Kaden and Rylie kissed each other, it started out that they kissed each other on the cheek and then escalated into they kissed on the lips. It was suppose to be a secret and he wasn't suppose to tell but the secret got out! I didn't really know how I felt about it at first. I was kinda shocked! I'm sure it was totally innocent and playful but I got some strange feelings! I don't want her doing that! And then a few days later Mike took the kids fishing with this same family and on the way home they saw Kaden and Rylie holding hands for a little bit. I think it's adorable but at the same time, really don't want them doing things like that! I just know we're going to have a hard time with this gorgeous girl when she's older so I don't want her starting young! She still sucks her thumb like a baby, she can't be kissing boys! Am I being silly? Is it okay to feel a little weird about it? Or is it no big deal and totally adorable?