ANSWER - It is always interesting to consider magic meant especially for young children as a lot of the methods we would apply to an adult are not appropriate for health and safety reasons. Crystals are out as a young child may swallow them. Oils and incenses may cause breathing issues so are not appropriate. Herbal concoctions are DEFINITELY not to be given to small children.
All these limitations put quite a restriction on a witch. But I do have a few suggestions.
You could try a blessing with moon or blessed water. Moon water is simply ordinary water that has been left outside in the rays of the moon during the night of a full moon. As this contains no other substance than actual tap water it is perfectly safe to anoint the child with and you can use it to place a mark on their forehead while speaking your words of intent aloud with relation to the safety and peace of the children.
Cord and thread can also be used to good effect but again from a health and safety perspective we do not advocate tying anything onto or around such a young child (for older children a bracelet woven with love and intent can be a powerful charm but you cannot run the risk of it coming loose on babies and getting into their mouths). There is nothing to stop you weaving or plaiting a loop or witch ladder though, providing this is secured safely out of reach of the child. This is an excellent way to transfer your intent into a physical object that can remain in situ to bring comfort.
You might want to take the idea of cord and thread even further – this site shows you how to make a dreamcatcher from scratch. The concept of catching the good dreams is popular in many cultures and also has the advantage of being something visually interesting for the child to look at. From a spell point of view you can weave the intent into it. From a practical point of view you can hang little bells from it which may help the babies to drift off to sleep.
As with all magic involving children it is important to obtain the consent of the parents and ensure they are happy with anything you are doing. This is an important mark of respect.