Could a first molar eruption cause enlarged submandibular node in a child? the node is 1.7 cm w/ bening features in us ('"reactive node").

Yes, but...: A molar eruption, mouth sores, gum damage, or anything else in that part of the mouth can lead to an enlarged node nearby (such as submandibular or neck nodes). However, the more common causes are strep throat and viral infections in kids. If there is nothing wrong with a child, and the only thing is one enlarged node, some doctors just recheck it each month for 2-3 months.

Yes: Eruption can cause enlargement of the lymph nodes. Nothing to worry about.