The regulations introduced on 31/03/2020 mean your child's legal stay is extended ex lege until the end of the 30th day following the cancellation date of the epidemic state/epidemic emergency (the last of the epidemic states declared). The legal stay is extended automatically for this period - it does not require any additional documents or a new residence card.
If you plan for your child to stay in Poland after this period (i.e. 30 days after the date of cancellation of the epidemic state/epidemic emergency (the last state declared), you should apply for a temporary residence permit on behalf of your child. The application must contain a copy of your child's passport, birth certificate with translation, a document confirming residence and your child's insurance registration. You can also include copies of parents' residence permits. Stamp duty payment will be also be required to begin the procedure. The payment link can be found HERE. If your child is over 6 years old, we will also ask you later to appear with the child to submit their fingerprints.
Details of the procedures regarding temporary residence permits for minor children can be found HERE.