1st layout with my son, new post with specifics :)

Father/son layout.... That is the life's blood of this hobby. So first things first. Paper and pencil.First- Supplies needed for the table and cost. Second- Design a track plan. Third- draw up the wiring for the track plan. Carpentry,drafting,electric and design. Aways have a plan,it most likely will change a bit as it moves along but thats what happens when you build a Model Railroad.  GOOD LOCK TO YOU AND YOUR SON.

OK, Tunnels, inside and out...

Inside is left open by most of us, at least after you get a foot or so inside.

this lets you get a grip on any derailed equipment in there from underneath or the back side or combo of both.

Outside and entryway is done many ways.

A portal surrounded by whatever mountain material you prefer always works.

Some folks stack broken ceiling tile and glue it up for shale looking rockface.

Trick on that: Always put in a slanted piece at the bottom to tilt the rock layers a bit, they are rarely perfectly level

Some prefer the Plaster cloth over cardboard or wadded newspaper method, that also makes interesting terrain and you can put foliage on it.

For the inside of the entryway, some folks crinkly up heavy foil and paint it black or dark brown. This gives a rough rock texture to the entry.

Some use foam they stomp on while laying on gravel or such for the same effect.

It's all up to your imagination. I have used an empty soda 12-pack (Fridge packs) taped to the back of the portal. It's cheap and if not overly durable, easily replaced.