I am so sorry. It really sucks to have something like that happen. I am afraid it sounds like pesticide poisoning. Often times the nursery does not spray the plant with pesticides but when the plant is potted they add a systemic pesticide in the soil. These systemic treatments are long lasting. So, the nursery workers are often not aware of what is in the soils. I don't know if this is what happened in your case or if the worker was knowledgeable. There have been studies that show that this type of pesticide (Systemic) is safe for adult butterflies who nectar but not for caterpillars. Imidocloprid may or may not cause death in caterpillars. Spinosad specifically lists caterpillars as one of the things it kills. It is applied systemically to control spider mites, a major nursery pest. The type of death sounds like what your caterpillars are going through.
http://www.livingwithbugs.com/PDFiles/mfs13.pdf

Again, I am sorry. It is hard when you try to make sure everything is right and it still goes wrong. We have all been there.

-Elisabeth

Here is a link that might be useful: Imidocloprid use

thank you for the information! I am hoping that they didn't eat enought to kill them, one of them seems to be doing better already, he's not excreting any greenish fluid anymore and his color is back to normal, he is moving around now too, and not falling over anymore, i am keeping their box completely clean and removing all their droppings as soon as i see them, i read somewhere that this may help, the other one still looks greenish, but he is moving around a little more, he is now able to stay upright some too, instead of just curled up on his side, I hope they can make it, I feel so terrible for them, they haven't had an easy time, almost from the day I saw them, there has been heavy rain everyday except for past day and a half, and wind, they made it through all of that and now they are having to deal with this! one of the other caterpillars actually made it to pupa stage, sunday, it was amazing to watch the chrysalis form. but yesterday evening, I checked the tree, and it was gone, the little strands of silk were there and just dangling, i guess a bird got it, I don't know. I went ahead and the last two inside as well, they both seem healthy, and i don't think they ate any leaves off of the new tree, so hopefully they will stay healthy and survive and become butterflies. Do you happen to know of anywhere online where I can order host plants for these caterpillars that would be safe to feed them? thanks again for replying, I really appreciate it!

The larger caterpillar is now eating, just a little, but he is actually tearing off pieces and chewing, I hope this is a really good sign, and that there is a possibility they will make it!! the other one is still a little greenish, but his color is looking more like normal, and neither of them are releasing that green fluid anymore, I am trying to keep them kind of hydrated by taking moistened q'tips and wiping around where they are, since they haven't been eating leaves to get any hydration . I read another post where someone said they have known Monarchs to survive an accidental poisoning that sounds similar to this, I hope these guys will too!!