Despite it being a literal child teasing him, a literal grown man, Sans can't help but flush all the same.

"Kid, you have got some wild ideas. I think I liked it better when you burst into tears at the sight of me."

Followed up by a wink, just to send home that he's joking. Hopefully it's not too soon to tease about that. Sans had a bad habit of underestimating the too soon limitations.