There seem to be two camps to layout design.
Roundy-roundy, great if you want to watch trains go 'round.
Point to Pointy if you want to operate and switch cars and all that.
Both are a matter of taste. I discovered that Roundy-roundy isn’t for me. But the Grandkids loved it when they were small. I had them operating my trains in the backyard from about the age of three. I think it’s great that you are going to have two loops, to run 2 trains at once.
Don’t be surprised if there are boy trains and girl trains. My little granddaughter, now 10, still calls the loco she used to run when she was three, Emily.
If you think you personally will be taking this hobby beyond the ‘only for the kids’ stage, than don’t put a lot of heavy lifting into your layout at this stage. Just minimally level what you have to to get the trains running ASAP. The ASAP part is probably the most important thing. In a couple of years you may decide, as I did, to raise the whole thing to waist height or so, maybe on benchwork, or maybe on serious earthwork & hardscaping, and you may then want to go Pointy-pointy with your layout, again as I have done. I am glad I made these two changes. I love operating, and I now have a small group of fellow operators. How one runs one’s trains is a matter of personal taste and priorities however. Most garden RR’s that I know are Rounders.
You’re going to have a lot of fun, whichever way you go, especially with kids in the mix!
I just wish it was warm enough up here in January to get out there and work on projects like yours! Won’t be able to do a thing 'til late April - and then only if IF the weather is warmer than usual. Usually it’s the end of May. :-((