Susanna:
Hi and congratuations on child # 3! I found myself in the same situation recently, and now have two girls and a nine month old little boy. I breast fed all of them and only mixed formula with cereal at about 6 mos. Still BFing the baby - he won't take a bottle, just chews on it. Aargh! it's exhausting, but worthwhile I know.
My 6YO was diagnosed this winter after much strange and uncharacteristic behavior was displayed at school. We have since learned of a family history of tics/OCD on my husbands side, and this explains my husbands obsessive nature and some other small things. We now see some of what (looking back we can identify as early warning signs with our oldest) in our 4YO. She has unusual extreme fears and is crossing her eyes when tired, etc. So, I am new to the knowledge that my child/children have tourettes. However, from what I understand, this is a NEUROLOGICAL situation. Certainly some foods or allergens may adversely affect anyone, however, if in fact your child is wired a particular way - that is simply the way they are wired. So I don't believe wether you breast feed or not will change the way that God has created their little brains.
Having said that there are things that absolutely affect my daughters behaviors. Being tired, stressed, hungry - we can see it on her face when she is any of those. A strep infection brought out some really strange things and a later ear infection saw another upswing in her movements. I have trouble calling them tics, because I understand tics as rapid movements, and my daughter displays more "complex/deliberate-looking/compulsive" type movements and fixates on thoughts etc. Still she at times is socially overwhelmed.
From what I have read this is a male dominant condition, so I too worry that my little guy might have a hard time of it. My girls are already noticably socially different and seem unhappy and overly senstitive/emotional much of the time.
Bottom line - I think BF is great for giving them a great start, but remember - food senstivites do not CAUSE the tourettes but may (as with any other bodily irritant) efffect the child's condition. Comments on this opinion are welcome. Thanks and good luck with the pregnancy!