Shy dog and kids?

I would like some advice on working with a GSD and small kids. I've had my Kenya almost a year. When I first got her she was a little skittish of a totally new environment and particularly leery of men. We've worked really hard on this and she is now a totally different dog. We did not push her at first; gave her time and space to come out of her shell and tried not to be too overbearing with affection too soon. She will always be a one-person dog that prefers women, but the "men" problem isn't really an issue anymore. Now she will greet strangers, she is very well-mannered and reliable in public, she will approach men on her own with her tail wagging. I've taken her so many places to work on her trusting me (and me trusting her) and to meet tons of new people. Besides her titles, she has passed the CGC three times, passed the Therapy Dog Int'l eval, passed training and an eval for a local therapy group, and will hopefully pass the TT in a few weeks. She is not dog or human aggressive at all (never has been as far as I know) and she is not scared of any noises or anything like that. We did a gun fire test at our SchH club and to prepare for other tests I've done stuff with my trainer like having "strangers" do weird approaches, grab at her, open an umbrella in her face, etc. She has no issues with any of these kinds of things. She used to jump at strange noises, but through a lot of work I can drop a bucket of rocks at her feet, open an umbrella in her face, etc. Mostly I've worked on ignoring/not coddling any nervous response while making positive associations with these weird experiences (giving treats and praise during gun fire, initially having men ignore her and treat her whenever she initiated contact, letting her play little games with me when the umbrella opens, etc). Now she's pretty unflappable!

The issue is that we haven't really worked with small children. It's not an "issue" I guess but I don't want it to become one. My youngest cousin is 4th grade so I don't really know any little kids (like ages 2-5). We meet plenty of adults in public, but not really any toddlers. I want her to be as well-rounded as possible.

We live in a duplex and on the other side is a little boy thats 3-ish. Tonight I had my dogs out and I was grooming Coke. The boy asked if he could pet the dog and I said yes. He came over and pet Coke. Coke loves everyone, always has. He has no confidence issues with people so that went fine, and I was squatting next to Coke and holding his collar (mostly to prevent Coke from licking the kid's face). Kenya was lying off to the side. The little boy started to go towards her. I stayed very close and I could see Kenya was nervous b/c this little boy approaches almost like he's scared of the dog, then he reaches high above the dog's head and tries to pet the top of the head. I could see Kenya's mouth close, lips start to curl, and her eyes get big so I said, "Don't pet that one, she's grumpy right now and doesn't want you to pet her" and he came back to Coke. When he turned to come back, Kenya did one of her little air snaps. She hasn't done that in a loooooooong time (I don't think she's done that since my FIL stupidly ran up to her and tried to hug her, this was while we were still working on the "men" issue several months ago). She did NOT bite the boy, she just curls her lip and does a little snap in the air. Of course I was close and paying attention so I had told the boy she was grumpy before she got too scared and he left her alone, didn't even notice that Kenya was tense and immediately left her alone as I asked. I commanded Kenya to go lie down by the back door.

It seemed like the way the boy approached, b/c he was so small and more cautious, is what made her nervous. Also he did the reaching up over the head thing, which she has never really taken to. If anything still upsets her, it's that. I use her to teach people how to pet a dog (when people ask to pet her I have them rub her chest or scratch her butt). After the boy played fetch with Coke for a while I took the dogs inside and fed Kenya treats while I pretended to do weird reaches directly over her head, but of course she couldn't care less about it when I am Her Person. Again, she did not growl, bark, bite, lunge at the child, or turn and run away. She tensed up, I took the kid back to Coke (who I was holding the entire time), she snapped at the air as he left, and I asked her to go lay by the backdoor while I showed the boy how to pet a dog (rubbing the chest, scratching the butt, NOT reaching over the head). Watching her get that tense look on her face is pretty much what I used to see when a man tried to approach and we've successfully dealt with that to the point where a man can fire a gun at her from 6 feet away and she won't flinch. I'm just not sure how to work on this because we rarely see a child that age and I don't expect anyone to be volunteering their toddler, lol!

I think the boy's approach and the reaching over the head was a big part of her being tense, but I'd rather prepare her for that than have to always be on the lookout for a loose toddler. Most older kids and adults have enough sense not to do that and act more natural in their approach, but you just never know and I want her to be OK with all sorts of weird stuff.

So how do you guys work on socialization around little kids? Where do I find a lot of little kids?