Can I be sued if the child has already been diagnosed with poisoning before living with me?

There are a number of issues you raise. One might be the statute of limitations and the age of the "child." Another might be whether your tenants had actual knowledge of the bankruptcy. But certainly the bankruptcy discharge can be raised as a defense.

Theoretically the debt is already in the Bankruptcy. Your Bankruptcy attorney ought to know this. In any regard you must assert your discharge as a defense in the lawsuit. You need an attorney.

You can add the creditor even after the bankruptcy is over so long as that potential debt existed prior to the filing.

You can be sued, but the filing and service of the complaint are likely a violation of the automatic stay and the claim underlying the complaint was likely discharged. If your chapter 7 case was a no asset case (i.e., you had no non exempt assets that were liquidated by the chapter 7 trustee and the proceeds distributed to creditors), the claim will be considered discharged except to the extent the plaintiff may have a claim for willful and malicious injury. If your case was an asset case, then no automatic stay violation and no discharge, if the creditor did not have either actual or constructive notice of the filing.

It sounds to me like the claim should be barred by your bankruptcy filing. Start by providing Notice of the Bankruptcy Filing and Discharge to the attorneys suing you. If you don't have copies, you may have to order them from the bankruptcy court. You may have to reopen your bankruptcy to add the creditor which would cost you some attorneys' fees and the filing fee to reopen.

Check with your bankruptcy attorney.