If you have an infant or unweaned toddler with you, milk is as necessary as water.
Children who only drink breastfed milk but don’t have their mother on the hike requires that you have breast-pumped milk in an insulated beverage container (such as a Thermos) that can keep it cool to avoid spoiling. Know how much the child drinks and at what frequency so you can bring enough. You’ll also need to carry the child’s bottle and feeding nipples. Bring enough extra water in your canteen so you can wash out the bottle after each feeding. A handkerchief can be used to dry bottles between feedings.
If the child drinks powdered milk, you can premeasure the powder into a bottle or two then by shaking vigorously mix it with water as needed. Again, bring enough extra water for bottle washing.
Don’t forget the baby’s pacifier. Make sure it has a string and hook on it so it connects to the baby’s outfit and isn’t lost.
Read more about day hiking with children in my Hikes with Tykes guidebooks.