I’m not sure any more condemnation is going to help , the OP is hopefully realising her actions were wrong now and needs to urgently rectify the current problem.
Most important now is that your cats are seen asap , yes it’s going to be more costly than if you had gone via a rescue centre in the first place , but it’s your responsibility to fix now .
The cats need to be seen , as PP has mentioned worming treatment now requires the vet to see the cat in the previous 12 months to dispense, so not a vet rip off just new rules .
Please don’t just wait 2 weeks and try to repeat the treatment, you genuinely may no longer have a kitten left if you do that .
What treatment did you use , if it was a cheap one like Johnson’s there are numerous reported side effects and it just doesn’t work properly. Panacur is the most common reliable treatment for kittens , but as yours has missed the doses from 2 weeks , you really need a vet . The vet will also be able to check for dehydration . Your main cat needs treating properly too , otherwise will continue to spread back and forth .
Have you also treated with a reputable flea treatment , because if they get a heavy flea load on top you will have a bigger problem.
I wish your kitten a speedy recovery.