Virtually impossible to convict on trespassing. It's not like they jumped the gate, they walked in through the front door, announced their arrival as themselves and the Secret Service, and perhaps an Obama aide, let them in.
The only thing that perhaps they could get pinched for is lying to a federal agent. Of course, that lie has to come during the course of an official investigation. I suppose a US Attorney could argue that the investigation was at the gate when the Federal Agent was determining if they were on the guest list. It's a stretch, to say the least.
They need to get them on something. It has become rather a thorn on the Secret Service side.
The Federal Agency has to be able to prove that they lied to the agent. This sound like a "he said/she said" deal to me and the only way that you could prove they lied is to get them to take a polygraph examination. Do you think they would voluntarily take such an examination?
Don't misunderstand me; I think they are lying too (but how does the Federal Government prove it?