I think this is one of the best small plans, and good for kids. All the components can be old school Lionel tubular track with manual turnouts that can be found very inexpensively on various sell sites. Put this on a 30 X 60 or so platform using either the foam roadbed or old school cork over a painted green sheet of 1/4" plywood supported by a frame made by 1X3's with cross stringers a foot apart. Put casters in the corners, and it will roll under most beds. You can remove tracks in the middle to make it less complicated, and add decoupler sections, but a transformer with a fixed output would be needed for those for best results, and those too can be found on the cheap. You can switch cars, park trains or locos using E unit neutral or a section of track isolated with a simple toggle switch to turn it off. The plugging in of the transformer into the wall outlet would have to be an adult monitored thing if the child is too young, or replace the plug on the cord with a hospital grade plug that is a large barrel shape that is easy to manipulate and plug in and out without getting near the terminals. You can delete the turnout and track going off board, or use it for an add on fiddle yard of just a straight piece of track on a board to give the trains someplace to go, or someplace to come from.
I forgot I had some pix of building this plan. I used an very old train board I built for my boys 35 years ago for an HO GI Joe train set, and expanded one end. The stringers are 1 1/2 inch tall and that is 1/4 plywood on top. I used cork roadbed just because, and sorted a box of old Lionel track I had to get the best pieces with all brown ties. The turnouts I got off a sell site, wanted to use manuals. I added a Lionel trolley line which goes through the cross over. I opened up the electrical connections and rewired it so the center rail was separate, and added extra wires to assure solid connections since normally they are just metal tabs bent over. Why the white paper? Because I am frugal (cheap?). A roll of grass paper was too small for my plan and if I used 2 I would have almost a whole roll left over. Sooooo, I layed down the white paper, put the track down to mark where the roadbed needed to go, and secured that, then cut out the white paper with a razor knife for templates to cut out sections from the grass paper mat and glues them down. That let one roll do the whole layout. The S curve you see is due to the coal delivery business building I want to put there and the track would have been too close to the loading dock, as a minimal S curve made room. The trolley runs from one end of the layout to the other, with a passenger platform on one end, and a hotel on the other, complete with its own business car siding. The other two sidings have a decoupler on one, and a operating section for a dump car on the other. The hotel siding has a decoupler section that has the electromagnet off set, which allows the business car to be dropped off, as a center magnet would not have been under the coupler on such a short siding. I use a Lionel Type R transformer to run it with an outboard reversing switch. I cut a hole in the plywood to make a small pond area with a wooden bridge for the track which uses the edge of the table to appear as a dam the water pours over. I added 4 legs to it, and included a pic of some really good leg hinges that lock securely when the leg is folded down. They were inexpensive on Amazon, and you get 8 with all the hardware. I just used the other 4 to make a slot car track table for my grandson. I take this with me when the wife wants to go to the beach, and set it up on the dinette table for something to do, otherwise I would go crazy, I am not beach person.