It’s not polite to hitch a ride on someone else’s coin. Truly, to the parents who allowed their much larger child to jump in with Roxy at the store, good grief! You didn’t even offer to pay for half the ride and didn’t introduce yourselves. Would have served you right if she had Swine flu!
Before you jump in and join something that is already started, ask first. Make sure it’s OK, that the child won’t flip out, or that she actually wants to share with your child.
Really, you thought this was OK? Friends and relatives are one thing, but total strangers? At least make a proper introduction before the coin is dropped in. This lesson will hold true throughout your adult life.