So I spread a bag of Tortoise Supplies testudo mix, but I'm not sure if everything growing is from that. I wanted to see if any of these mystery plants are edible.
It's a little hard to tell. Can you confirm a few things?
are the leaves coming out of the stalk "opposite" each other? Or do they "alternate" up the stalk (first on one side, then the other as you go from bottom to top)?
the edges of the leaves ("margins") look pretty smooth to me - are they? or do they look like, for example, a serrated knife?
if you crush a leaf, is it "aromatic"? does it "stink to high heavens" ??
is the stalk square? or round? if you snap it, does it have a milky sap?
any flowers or buds yet? If so, can you get a close-up (from the front and from the back)
how big is this plant in relation to any that you know came from the Testudo Mix? Your 2nd picture leads me to believe this didn't come from that Mix (you've got a lot of it there and all in one place)
I did a google image search - the first pic came back "nightshade" - that's not necessarily bad (tomatoes are in the Nightshade Family), but then again it might be (so is Atropa belladonna - deadly nightshade!) I wouldn't feed it to your little one until you know for sure what you've got.
Do you have a Cooperative Extension in your state? I have found them to be very helpful identifying local plants. You might also try looking here: Reference List for Maryland Wildflower Identification. On the right hand side of the page, you can browse through 468 pictures (if you like doing that sort of thing!) Or there are many places listed that may be able to help.
Tortoise Supply Company intentionally doesn’t say what seeds are in the mix—trade secret—but I can’t think of a likely seed that would grow into what you have. It’s also growing next to grass of some kind and there is no grass in their testudo mix. So I don’t think it’s anything you planted on purpose.
It puts me in mind of young willow herb but that’s a real guess on my part. I wouldn’t guess further without seeing some flowers, even if they are tiny and drab.
It doesn’t “look” dangerous to me but that’s not good enough information to go on. I’d avoid it until such time as you really know what it is. There should be plenty of good things to feed from your seed mix in the near future so no need to guess.