The spare outfit in a bag can be months older than the clothes your child wears every day. Because it is rarely used, it is easy to overlook until a change is needed. Check spares when you change the everyday wardrobe, not only when the bag is empty.
Lay out a complete change appropriate to the outing and weather. Include the pieces your child actually wears, rather than assuming a single extra top is enough. For a toddler, consider whether the outfit is easy for them to help put on and whether the shoes will still work if socks need replacing.
A spare outfit should solve an ordinary clothing problem. Choose comfortable pieces that fit and are easy to handle. An elaborate outfit saved for photographs may be inconvenient in a small restroom or during a hurried change.
Keep clean clothing together in a washable pouch or other suitable compartment. Have a separate way to carry used clothing so it does not soak or soil the remaining supplies. Keep any bags inaccessible to young children and remove used items when you return home.
The number of changes depends on the child's recent needs, the outing and access to laundry. A short visit nearby and a full day away should not automatically receive the same quantity. Add a reasonable spare without turning the bag into a collection of every possible outfit.
If several caregivers share the bag, make the location obvious and restock after use. A simple note that the spare bottoms were used is more useful than discovering the empty pouch on the next trip. Consider a small spare item for the adult too when that would address a recurring need.
At the next size change, remove clothes that are too small even if they still look new. Unused does not mean useful. Put the current spare back in the same accessible place and check it again as the child's needs change. The best backup is the one that fits the child in front of you.