OK so does the latter carry a "heftier" sentence?
I guess what I would look for is leverage LE has to get the parents to cooperate (if they simply are not providing LE with everything)
You'd have to look at state law to see how they're treated but if you consider the difference between the two, A&A actually assists in the commission of the crime whereas accessory is more like an obstruction of justice. So A&A could be seen as worse.
The federal sentencing guidelines treat an A&A conviction as being a "principal" to the crime - and can be sentenced in the same manner that the crime itself is treated. Whereas accessory is can be a lower offense for serious crimes like murder.
But yes, any meaningful threat of an accessory charge brings leverage to the table. An actual arrest on accessory brings a lot of leverage. Of course, if they don't really know where he is and have told them everything they know, there's not much more to get out of them. So it's up to the detectives to use the other information and evidence to look for any gaps or deviations in their story to them.