A passport is required for international travel, including for children & babies. Many countries (including Mexico, Costa Rica, St Lucia and throughout Europe) require that passports be valid for six months AFTER RETURN DATE OF TRAVEL.
As of July 2021, Passport processing times are extremely delayed due to increased travel demand. To check current processing times, please visit: https://travel.state.gov/content/travel/en/passports/how-apply/processing-times.html
If you have additional questions, please contact me.