You don't say how are you going to teach Latin to your
child, and how is he going to practice it. I believe that Latin
is not a practical language, and less for a child.
All previous answers assume that the child can learn only
one language. Children can easily learn 3 or more languages
as long as somebody is speaking to them in each of the
languages on an almost daily basis.
The best combination is for mother to speak one language,
father to speak another language (looks like here should
be English and Spanish) and another person should speak
the third language. This person could be a relative, a
babysitter ... In absence of this third person, the best third
language could be Esperanto, that could be learned, and
used, by any of the persons in contact with the child,
including the parents.
It is not necessary to emphasize English, nor even use it.
They will pick up the local language from TV, friends and
school. The more languages they know, the more incentive
they will have to learn English, the local language.
Esperanto will be a good choice for the third language,
because it is easy to learn, same for the child and for its
parents. There are enough materials on the web, including
video-animations very much liked by children.
And the next problem with learning languages, is to have
a way to practice it. People that learned Esperanto want
to speak with other Esperanto speakers. That is not
common when you want to practice another language.