Peteeng... Pretty sure we were both typing at the same time. I didn't read your post until after I posted mine.
Your advice is very very very helpful. I have actually been debating concrete or plywood but kept having 2nd thoughts. I read all sorts of posts about concrete cracking, shrinking or not working with different materials. I was looking at the countertop stuff but will definitely look at the concrete grout. With that said I have a bunch of plywood bed slats collecting dust and I'm starting to think they might work perfect. I will still throw in some steel and aluminum but for added "bulk" the plywood sounds promising. I could always go back with grout later.
You hit the nail on the head as far as my goals and agenda. I'm not overly concerned about the little details. Its mostly a fun project to get her brain thinking and to spend time in the shop with pops. She likes watching all the YouTube videos of people building things and that has been some of our motivation.
Really half the fun is her being able to rummage through the scrap piles and kind of day dream about all the different things we could do. I remember as a kid wanting to take everything apart or constantly trying to make something out of leftover wood and metal (I'm 40 and still do this lol)