"make pretence" is a phrase meaning to pretend. If the pretence is vain, it means it was unsuccessful. So the first line you've bolded means that they were pretending unsuccessfully, but pretending to do what?
In the second line you've bolded, anastrophe is employed: changing the usual subject-verb-object order for the poetic purposes of rhyme and rhythm. "Our wanderings" is the object and "to guide" is the verb.
So you could rephrase the whole thing as:
> while little hands pretended unsuccessfully to guide our wanderings.
It means that the children's efforts at the oars may be trying (or pretending) to send the boat in particular directions, but in reality they are moving in a more uncontrolled way across the water.