We heard the same things and did this for years for myself, my husband and our son. We got no relief at all from allergies. We also took high-end, single ingredient drops he recommended. Lost a lot of money, kept the allergies. We did get relief from some back/neck ache problems, and one of us learned that one leg was shorter than the other so a lift was added to one shoe. Those things helped.
I did other supplementation and got rid of all my symptoms of allergies, and I no longer take any medication and don't get shots. We've had a couple of bad spring allergy seasons in the northeast, and I'd say I've had about 6 uncomfortable days in 6 years. I manage those with more food science answers and occasionally with an over-the-counter allergy aid just for those few bad days.
I've been trained in food science and nutritional epigenetics, and I work as an educator in these areas. I have seen many people improve their immune systems, and we use independent clinical studies, university researchers, etc., as the basis for this. I do my training with many chiropractors, massage therapists, nurses, pharmacists, and physicians - all because they feel that their practices aren't sufficient to deal with allergies, immune system support, autoimmune condition reduction, and so on. A colleague of mine did the allergy tests and was allergic to everything except 4 trees, and her 1 year old was on a nebulizer 3 weeks out of 4. They are all symptom free now, and didn't do spinal adjustments for it.
Good chiropractors will tell you that you should get relief (maybe not total, but significant) after a few sessions. Any chiropractor that tells you to keep on coming back forever I think should be questioned.
I'd really question anything you're going to give a child. I really focus heavily on where things are made, under what conditions, and whether they are safe. I don't use things that have warning labels on them, which many supplements do.