Time Portals

"Do you think dinosaurs still exist?"
This goes back to our "all things exist at the same time - there is linear time only in our perception..."
I always get a little "is this a trick question?" when he asks me.
"Um.... yeah, I'll have to say yes."
"How are we going to get back in time?"
"We'll have to build a time machine." (as discussed previously.)
"Yeah. Rocket scientists know how to build time machines. And map scientists. We can ask a rocket scientist and map scientist to show us how to go back in time. Daddy will be on the t-rex duty (I had already said that I don't want it - I don't see them being a particularly friendly bunch.) You'll be on the ornithomimus duty (which I was graciously given later after my t-rex objection, they are little, and vegetarians. Perfect!), and I'll be on triceratops duty. Then we'll just go through the time portal."
"Well, wait a minute, do you want a time portal, or do you want a ship? If we have a machine, or ship, it will have to be large enough for our creatures."
"A time portal. With a time portal, it's easier - you just have to walk through."

So now we need to investigate to see if anyone has any ideas on How To Build a Time Portal...