The seed allergy may be a cross reaction with the soy. My son also was allegic to all seeds all nuts and soy until age 5. He's good for soy now opening up many doors but our new allergist told us sunflower seed is very often a false positive with soy reaction. I think most of the seed tests ate not specific enough. My son does react to sesame but we've eased off restricting some other seeds (now if we could just get him to eat them). If you are comfortable, test a little patch of skin inside the arm with sunbutter. Or talk to an allergist if you are not confident to do it yourself. Many many of the allergy "positives" are not. As you become more comfortable with how your kid reacts you will feel safe challenging them.
My kid has umpteen allergies and an even longer list of things he refuses to eat but he is thriving off mostly meat and fruit and veggies.
Hemp is seed so make sure to do a safe challenge first before giving them hemp milk.