Ah right. True. But:
BBC purposefully used old footage to show previous attacks
or
The child was moved to another hospital where the BBC just happened to be filming
or(as you said)
Being in a hospital for 3 days isn't unusual.
And
None of this matters or is even relevant since there are people suffering while we don't bother to address the actual situation, but instead focus on "is this fake"?
Twitter is faster than BBC so that's probably it. But I'm curious what those tweets where saying and who posted them.
But if the Syrian army was really using Phosphor bombs everyone would know about it without a shadow of a doubt. They would have targeted actual Rebels positions and made kills. So why no footage of burnt rebels?