What do I give a child who’s allergic to both cow’s milk and soy?

Unfortunately, some children are soy protein intolerant. As it happens, this is more common in kids who are allergic to cow's milk protein. You might try using Alimentum or Nutramigen (protein hydrolysate infant formulas) as the milk for the next 2 weeks because a child is much less likely to be allergic to the protein in them. If they work, you can then experiment with other sources of calcium, protein, and fat for the future (perhaps rice milk).