K.L.
I don't think it would be imposing to let her know you'd prefer to stay. You could simply explain that you would rather give your child a chance to get to know the new friend and friend's mom with you there. Then, if all works out, in the future you could plan to leave for an hour or so, if you are comfortable doing so.
I am guessing the mom was probably just being polite and offering you a break and didn't mean any harm by the suggestion.