i.e. Under his care and inspection. See Genesis 39:22. And it must be noted that these words belong both to the Merarites here, and to the Gershonites, Numbers 7:7, because both of them were under his hands, as is affirmed, Numbers 4:28,33.
Related Commentaries of Numbers 7:8
Numbers 7:8
8 And four wagons and eight oxen he gave unto the sons of Merari, according unto their service, under the hand of Ithamar the son of Aaron the priest.