Your sentence consists of two parts that in grammar-speak are called clauses. A clause is a unit of grammatical organization next below the sentence in rank and in traditional grammar said to consist of a subject and predicate. The subject of a sentence is who or what the sentence is talking about and the predicate is what the subject is doing or what is being done to it. A clause can also be viewed as a complete thought that can stand on its own in terms of its meaning (there is also something called dependent and independent clauses, but that's a topic for another discussion).
In a multi-clause sentence, which is what we have here, saying that a sentence is in the passive voice or in the active voice does not make a whole lot of sense since the concept of voice, in that case, can only be applied to each individual clause, not to the entire sentence.
The active voice describes a sentence (in the case of single-clause sentences) or clause where the subject performs the action stated by the verb. In passive-voice sentences or clauses, the subject is acted upon by the verb. Take a look at these two examples:
I am watching her. (active-voice sentence)
She is being watched by me. (passive-voice sentence)
Can you see the difference between the active voice and the passive voice? In the first sentence, the subject I is the doer of the action because I is the one who is doing the watching. In the second sentence, the subject now is she which is no longer the source of the action of watching. Instead, it's being passive with respect to the action because the action of watching is now being preformed on the subject. Let's now apply all that newly-gained knowledge to your sentence:
First clause: the pediatrician will want to see whether (active voice)
Second clause: the grasp of your baby is being developed well (passive voice)
If you want to turn the second clause, which is at the moment in the passive voice, into an active-voice one, you need to make the baby the doer of the action of developing its grasp:
your baby is developing its grasp well (active voice)
So, the entire sentence would now read like this:
The pediatrician will want to see whether your baby is developing its grasp well.
Whether or not it's a good sentence is a different thing, but this example does answer your question.