I don't think giganteum grows in the Dallas area, only Fat Hen and berlandieri. Were there any oaks in the vicinity? Quercus muehlenbergii? Chinkapin Oak?

Yes. They look like tree seedlings to me too. There are acorns right there in your pictures. Bet you'll find one if you remove some soil from the base of the plants.

The leaf shapes are Identical. Every photo of Chenopodium gigantea Ive looked at shows the emerging new leaves a deep red color.

Hmm, or maybe it's the similar Goosefoot (Chenopodium album)? There was one growing in my yard a while back...

And that "acorn" is a Pecan, so I don't think it's an Oak?

OK. I'm not familiar with pecans in the shell. But I'm pretty certain it is a tree and not an annual weed. It is 100% not Chenopodium album. Leaf shape, habit and texture are entirely wrong. I would still have a look for an acorn.

As so often your photos don't show much detail. How about showing the stem and leaf arrangement?

OK, here's a profile view..

Oak treelets. Absolutely. Dig around and you'll find an acorn at the bottom of each one.

OK, they grew some more and they now appear to be some type of Euphorbia...like maybe Euphorbia cyathophora (Wild Poinsettia)?

It's definately Wild Poisettia. Those little leaves are going to turn fire engine red. Nice! Someone told me it's extremely agressive be warned. I like the native Croton species. I'm sure you must have at least a few of them down there.

Oh dear. A lesson in hubris for me. I got this one completely wrong. One of the problems with photos is that one cannot judge texture. Another id trick is the leaf tear. Both would have ruled out the oak idea. As would the lack of an acorn.