New here, worried about sick/hurt chick

I hate for this to be my introduction, but I have a question about my chicks.

We got 5 of them from a local feed store on April 1st, everything has been going great... Until about 2 days ago one of the babies stopped walking properly. The others are fine, but she is now not eating or drinking unless I put it in front of her. Then she tries,but its not much. She appears to be breathing kinda fast as well, we called the vet and he said in a couple days to put her down if she is not better. That it could be mereks disease and to send her body in to be tested. I have her separate from the other girls, I just feel terrible for her and hate that she is alone sick, so I try to hold her as much as I can. We have 4 human children, who have fell in love with their baby chicks. So I am hoping she can get well, but she is looking pretty sad. She is a little bit over 3 weeks now by the way.
Also we contacted the hatchery and they said pretty much that mereks is impossible for her to have at this age, and she probably got into something. These babies have been in the house and never outside yet, fed only what we give them. I just don't know how that could happen.

As I understand it the Mareks virus is everywhere and young chicks CAN get exposed to the virus. This is why we are in a hurry to vaccinate our chicks at 1 day old. Did you get the Mareks vaccine for your chicks? It's usually about. 50 cents for the vaccine per bird. Cost at 10 birds. The more birds the less the price usually. The hatchery usually offers it at checkout? Vitamin deficiency can also cause such symptoms. You can try giving a poultry vitamin with riboflavin for her lameness and see if any improvements.

The store we got them from said they were unsure if they had the vaccine or not. When we contacted the hatchery that they got them from they said they only do if ordered that way. So I am unsure if it was given or not, I was told I could not get a vaccine for just my 5 chicks as well. Thank you for the suggestions, I will try that.

You should not handle them very much as they are very young and can get injuried or sick from germs we carry on our bodies. Picking them up is fine after they are feathered. I do not pick my chicks up unless I need to, to clean pasty butt. Get some sav-a-chick or probiotics. Electorlites may help as well. As some have said vitamins would not hurt as well.

Oh thank you! I had no idea, we have been cuddling them a bit. I thought it would help them get used to us, I definitely will try your suggestions.

Quick update- The sick chick is finally walking around a little bit today. She seems so much better, I gave her the save a chick probiotics and electrolytes.
Now my other issue is last night one of our healthy chicks was fine when I checked on her. Then we check them again and she was dead! Then today a chick died right before our eyes! We have only two "healthy" chicks and one who is relearning to walk. So sad! We sent the ones who died suddenly to who ever tests them for illnesses.

So the baby chick is all better now, she is walking around and doing great! I got results back on the ones that died, they said they had metabolic bone disease and had rickets. I don't know how that happened, the food they are on is what I thought should be good enough... I have our last 3 chicks on medicated feed and save a chick vitamins and probiotics. They seem to be doing well, but the two that died suddenly seemed well also.