I think you mean, "dann", because "als" doesn't mean "then", but "than" instead. I think you're right, I just thought this form with "then" sounded more... english :)
It is "ist es" because the verb is always in second position in a German sentence. The "wenn" clause occupies the first position, so "ist" must come next.
Why "... it is a good ..."? Shouldn't be " ... he/she is a good ..."? Is this a kind of politically correct expression? In this case it should be "she" ... I think.
in german, "baby" is neuter, so they refer to a baby as "es." even though we don't use "it" to refer to babies in english, the baby's gender is not specified here, so that's the best we can do.
When doing English -> German translation, when the gender of a noun is not clear, Duo generally accepts either (e.g. Ich bin ein Schüler/Ich bin eine Schülerin), why should German -> English differ?