Oh no! That's so sad. I don't think 6 weeks is too young to free range if they know where home/coop is, and if they know your voice or the sound of the food container or something else to call them towards you.
With my chickies, I only let them free-range unsupervised when they know that my "here, chick-chick-chick-chicKEN!" call means food, and they at least show themselves, if not come towards me, when I call. Every day of their lives, I call it every time I feed them, especially treats. Sometimes I call it when they're just eating happily, so they associate my call with food. Then when they are in the yard, if I can't see them, I can call and they'll at least run towards me even if they're too skittish to be caught right then.
But I was opening the dog crate and letting them roam the yard at 6 weeks old while I went to work, because I didn't want them in with my big girls yet and the crate was too small to keep them in all day. I'd call them to the crate right before I left for work, and again as soon as I got home, to make sure they were close by. We have cats that come into our yard and the very rare hawk, but no major predators. This lot of chicks have been fine in the yard, but I don't know if we got lucky or if chicks are generally ok at that age.
I don't think you did anything wrong, it's just an unfortunate circumstance. If you get more, definitely train them to come when called, so if they do end up running into the woods, you can call them back. When they're little, they don't know to come back and roost at night. Mine are 9 weeks old now and even though the other chicks and chickens get into bed every night, there are often 1 or 2 naughty chicks that roost in the rhodedendron bushes instead of going to the coop. I found 2 in the run this morning when I went to let everyone else out and I told them I knew they'd been out all night and were very naughty chicks. They are also lucky they didn't get eaten...
