need convenience function for /entity/child

There are new services that replace the most commonly used queries to the deprecated entity query service. They are:

http://docs.synapse.org/rest/POST/entity/children.html
(lists the children of a given parent entity)

http://docs.synapse.org/rest/POST/entity/child.html
(find the entity with a given parent and a given name)

There is a convenience function in Python for the first, but not for the second. Need convenience function in Python (and by extension in R) to facilitate users stop using the entity query function.