This is a problem for me all the time! When I was a few years younger, it really bothered me, but I got use to it. Often kids just tell the truth, sometimes the truth hurts, but it is just the truth. That is the best part about kids, usually they aren't malicious, and when they are actually trying to be, it doesn't work very well. (i.e. I don't like subtraction, it's stooopid, you're stooopid because you like subtraction! It just doesn't really hurt. Sorry kids.)
Another thing you can do (besides getting older) is work with more kids! I find that when I spend time with hundreds of the little buggers, just 5 saying something that I really wished wasn't true isn't so bothersome. I tell my self that they are all perfect vessels of honesty, so the others just didn't notice!
Also eventually you may get a kid that says something mean to you in a nice way. I always get "you have a small head" I did not think my head was small until I started being around mass quantities of children, but whatever, they said so. Anyways, this one kid hands me his book to check out and said in so many words (we're ignoring bad sentence structure, rambling and a little bit of a lisp) "Do people tell you you have a small head"? Me: Yes they do! "Well it's okay because everyone is different and that makes you special." He was trying to make me feel better after insulting me a little. It was sweet!
Third, work with someone who has some kind of severe autism. Here are some questions I get repeated to me in a loop throughout an 8 hour day, and no longer bother me:
Q: "Do you drive?"
Internal Monologe: (Yes damnit, that is how I got here, you saw me drive here, and I answered this question 2 minutes ago)
A: "Yes I do"
Q: "Can I drive?"
IM: (Oh god I hope not.)
A: "No, you have to be 16 to drive, and you have to be able to read"
Q: "Would I catch the car on fire?"
IM: (What??? How? ....Maybe.)
A: "No, you wouldn't."
Repeat above on loop until distracted.
Q: Do you have a boyfriend?
IM: (None of your bee's wax kid.)
A: "Yes I do."
Comment, said with his eyes closed and hand out for a hand shake, a big improvement over the infamous boob hug that was previously standard procedure: "You're so pretty, you're a good girl."
IM: (EEEEEEEEEWWWWWWWWWW ew ew ew)
A: "Thanks."
Comment: "You're my girlfriend, we're going to get married."
IM: (Out of your league kid)
A: That's not how those things work, you have to ask someone to marry you, and that's not appropriate for school.
Follow-up comment: *confused face*
Comment: "Brianna is my girlfriend. I have three girlfriends."
IM: (Yeah I was just talking to Brianna, she was trying really hard to ignore you.)
A: Girlfriends are for when you are older, and you should only have one at a time.
Follow-up comment: *confused face*
Q: Are you pregnant? Do you want kids? Will they be annoying?
IM: (I see why you would think that....)
A: No.
Q: "Are you ticklish?"
IM: (I am so glad I always stand more than arm's length away)
A: "No. It's not appropriate to ask that at school."
Follow up question: "Will I get beat up?"
IM: (If you keep asking that guy in the park...)
A: "No, it's not what we do to our friends or adults."
Question to the kid sitting next to him: "Are you ticklish?"
(Sigh.)
Then there was that time where I was unemployed...
Q: "Do you have a job? Do you have a house? Do you live with your mom? Do you have a dog? Do you have a robot? Do you want a dog?"
IM: (Sigh.)
A: No.
(The school is next to a graveyard)
Q: Is your mom dead?
IM: (Not last time I talked to her.)
A: No.
Q: Is your mom dead?
IM: (Really, she was fine yesterday.)
A: No.
Q: Is your mom dead?
IM: (Maybe I should call her.)
A: No.
Q: Is your mom dead?
IM: (...)
A: Is yours?