Hi there. Your Wiemaraner sounds lovely :)
My Collie has the same issue with kids, any child under the age of 11 really. He's two-and-a-half, and hasn't grown up around children, so has little experience or understanding of their unpredictability, and the noises/movements they make.
I just keep him under my control around any children that I know will upset him, not allowing him too close to them. Fortunately my family doesn't contain any younger kids-I know it sounds extreme, but I would rather remove him from the situation completely than stress him out and risk him snapping and being branded dangerous. Some people may suggest gradual exposure to children as a desensitisation method, but I don't want to put my dog, or any child, at any risk.
Revisiting what I have written, I see that it isn't so much an answer as sympathising, sorry! But I do know how you feel; I am sad that Jack isn't a more approachable dog, but that's his nature and I wouldn't want anyone harmed so I do what I feel is best for his and anyone else's safety.
Good luck :) Lisa.