When my twin daughters, who are really slim (i.e., have never been "on the charts" for weight!), turned one, our doctor said to start with cow's milk. I was still nursing them although they had eaten solids beginning about 8-9 months. I initially did not want to do milk at all (although like another poster, they had cultured dairy from the beginning -- whole-milk yogurt and kefir in our case).
We decided because of their weight to go ahead and try the store-bought milk, which in our case worked OK by some standards. By this I mean they didn't have any terrible reactions per se, but they did have more clogged noses with yellow/green mucous, and diapers that were pretty awful. (Not well formed stools, but yucky-messy.) In retrospect, I think those two situations are symptomatic of a milk allergy -- a lot of my friends take those symptoms for granted as sort of standard, but in my opinion they are not normal -- by the time they are eating healthy solids, stools should be more solid, and I think a lot of people assume that children have "colds" when in fact mucous can be food-generated.
We moved and were able to start the raw goat milk, which is why I say the above -- almost overnight, no more nose gunk, and much healthier-looking diapers. (I have a friend who gets raw cow milk, and when we have visited her, my girls have done well on that, too.) It was actually pretty astonishing to me, and I felt bad that even though the diapers/gunky noses did not keep them down, that those things -- constantly fighting the effects of pasteurized milk, in other words -- may have kept them a little sub-par for several months (we started with the fresh milk -- a term I prefer, because it is really a "fresh" v. "processed" issue, rather than "raw" v. "cooked," so to speak -- when they were 16 months).
The only other thing I have thought that might explain the difference from the aformentioned symptoms being present to not being present is that the time span involved was May-September (when we used processed milk) and that perhaps the noses/diapers were affected by pollens. However, this spring they have not been sniffly at all, so I am inclined even more to attribute it to the milk switch!
We don't vaccinate, which I also believe is helpful in allowing their immune systems to function in top form, so I am never sure what to attribute to no vax, to raw milk, or to some other decisions I have made about feeding them (following the Weston A. Price suggestions rather than the standard American baby diet, extended nursing, etc). However, they do seem to have been healthier than most other kids we know in the sense that they do not get colds, ear infections, etc., and have never had diarhhea or other serious stomach problems. (We also don't have older children, which I know makes a difference.)
If I have another baby, I would either only do raw milk or none at all (regardless of weight issues). I am not one who believes milk is necessary for calcium and other minerals -- it's more about grass-fed and the nutrients that come through the milk via grass (if we run out of raw milk, sometimes I will buy pasteurized milk that is from a local grass-fed dairy). I guess my feeling is, why bother with processed milk? (I'm not trying to be dictatorial about this, but I do have a gamut of experience at this point!) I would (with another baby) stick with the yogurt and kefir -- one of the first foods we fed to the girls was a very creamy version of the cream-on-top yogurt, and they liked it and have had few stomach problems.
Sorry to go on (I had a visitor during this post and sort of lost my train of thought!) -- this issue is dear to me as I feel so bad for kids with these low-level allergies that keep them (like they do us) running on half-empty. I have said recently on other forums that I'd rather be good and sick for a week than to be half out of it for months with chronic illness and I get so sad when I see these people feeding absolute junk to their little ones and then having to go to the doctor, get on the Antibiotics
cycle, etc. I am always grateful for the informed and interested viewpoints on this site and forum, which certainly make me feel less lonely in some of my choices about feeding my kids!
Hope you all are well.