While it appears that the day care center was negligent in letting your child ingest some type of medication, you say that your child seems “fine”. You should take your child to the doctor immediately for examination to rule out any injury. You have no medical records to support the poisoning, nor does it appear your child was injured at all.
Unfortunately, without any injury you have no negligence case.
DISCLAIMER: David J. McCormick is licensed to practice law in the State of Wisconsin and this answer is being provided for informational purposes only because the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship.
Assuming that you have had your child checked out, unfortunately a lawsuit would probably not bring you any compensation as there apparently was no injury to your child. You could hire a lawyer to bring a claim against the day care's insurer and they might pay something in the way of nuisance value.