Fix my lack of common sense!

Hey TQC,

I am a friendly person, and I love to meet new people. My friends tease me that I can make friends, or at least conversation, in almost any situation. However, I have noticed that sometimes guys (that I have no intention of flirting with) mistake my friendliness for interest in them. It's been a problem lately, so I was wondering if anyone here has had the same problem and if so, what did you do about it? 

This last time I was put in a bit of a awkward situation, because a guy with kids came in to my (kid-friendly) workplace, and I was nice to his kids ( and barely spoke to him because the kids were so chatty). But his kids kept saying that I was nice and we should all be friends, and since they were so young I was like "yeah" because who says "no" to cute little kids? But then the little kids came up top my desk without their father their and were all happy and asked for my number and like a total idiot, I gave it (my mind blanked out at the weirdness, give me a break). Well, I assumed that their father would have the good sense to take the number and throw it away, since who wants their kids to hang out with a total stranger, right? Wrong. Now he's texting me and I know he wants to meet up. I don't remember even talking to him much! How do I handle this? I have ignored his couple of texts. 

tl;dr: Like a total idiot I screwed up by giving out my number in trying to be nice to kids and now their (divorced) dad is texting me when I'm not interested.