I really, really appreciate your answer.As a Spanish citizen, your child will be visa-exempt and can come to Canada as a visitor with you and your wife. You can then submit the sponsorship app. The biggest consideraton is that depending your intended province, your child may not have access to provincial health coverage while a visitor.
You can't include your child in you current app.
I would only have one more question: if we go to Canada and begin the sponsorship, would the child be able to stay with us the whole time, or should he leave the country at any point?
For example, the eTA will be valid for 6 months, but if the sponsorship takes longer, I don't know if they child will be able to stay in Canada or not.