Are you the only adult? If so, I am pretty sure they will let you all board. If there is another adult, probably not. It all depends on how many preboards there are. When I flew in November there were 8 wheelchairs and 4 other people for preboard so they were strict about only allowing 1 other person on.
But I don't know for sure. But as a whole, SW has been very accommodating so if it's just the 3 of you, I can't imagine they would actually say no but since an 11 year old can travel as an unaccompanied minor I can't say with 100% certainty that it will be allowed. I would call SW and ask.