The time to lie is when you want your kid to go to a special class you need to be older for... like a computer design class they are 8 months too young for, or the older sessions for instrument lessons. I wouldn't lie at the playground. You don't want to send the message it is ok to lie. The playground is a kinder version of preparing you for school anyway ;-)
Although I did often follow my uncomfortable moments like yours with a comment about the girls gross motor skills. Like lady it is ok my kid is reading the stop sign because she just walked into a wall. Or you could go with bad sleeper or something to level the playing field - if you want to fit in.
Re: leveling the field. I used to do that, but have stopped because I found that DD (and now DS) take it as meaning the way they are (cognitively) is some how "bad" or "wrong" that I have to take drop them down a notch to normalize them to make other people more comfortable. I'm now working on giving a matter-of-fact but nice (I used to feel a bit defensive and it came out in my responses) answers to various questions re: my kids' abilities, age, etc. and *NOT* pulling in a comment about something my kid does poorly. I do however try to take a moment to think about what I've observed about the other child. If someone commented on my DD's reading ability in comparison to her age, I might say something like, "Yes, she loves books". And then follow that with something like "I see your DD/DS is really enjoying X! That is very cool!", for example.
