On further investigation we have found the field (skill_level) referred to by the Amazon response.
There are several versions of the Amazon product feed. These include product specific xsd files that describe the product type. You currently classify your products as toysbaby which comes from the includes xsd called toysbaby.xsd.
There is another xsd called toys.xsd in which the field you are looking for is present. However this xsd is not included in the product feed xsd that we are using.
If I had to guess, I would say you found the item type in the browse tree guide which is referring to the toy.xsd 4.1 A way around this might be to try a different item type for that item.
Unfortunately we don’t know which version the item types refer to. The only way of finding out is to feed Amazon as you have and see what their answer is!