I am going to let you in on a secret. Someone, a teacher/mentor, once told me. (Unfortunately I have a bad memory and don’t specifically remember who it was).
Always keep any extra pair of clothes in your carry on. If you don’t have a carry on then in your purse/small hand bag.
When traveling by plane, it can be pretty unpredictable. A child could spit up or spill on you. You could have a delay and get stuck without your luggage. There is always the danger of getting sick and having an accident. Yes, that does happen. Never to me but I know some who have.
If traveling by ground transportation: car, train or bus. This still applies. Normally we have a small bag in the back of the car with an extra change for all of my girls. And for us, if it’s a long trip.