Shouldn't it be "must be presented" to say this sentence in the passive voice? Is it still correct in grammatically saying just "present"?
Yes, you are correct. It is more natural to say "must be presented" instead of "must be present" here. It appears the speaker has used "present" instead of "presented" because of the latter half of the sentence, "if they themselves cannot be," as the correct expression is that people are "present", not "presented". Regardless, "must be presented" likely fits better in this context. The best way to express this sentence is "pictures of your children must be presented if they themselves cannot be present."