I'm going to send you over to look at the district web site and the policy. The school principal is the person who determines whether you can take advantage of the open enrollment option. I recommend that you get an application in as soon as possible.
Heritage and Trail Ridge middle schools are within walking distance. However, if your 11-year-old is enrolled at another school in the district, your student may be able to attend that school until you ready to be relocated.
As a parent who used to live close to the Longmont elementary school that was given the lowest grades in the city - I completely understand where you are coming from.
I have good news and bad news. The good news is that Longmont does have the policy to allow parents to apply at whatever school they choose to enroll their children at.
The not so good news is that you do need to provide transportation to that school if it is not in your district (a small price to pay for a quality education in my opinion) and you will also need to apply EARLY as the better graded schools are in demand and they cannot over enroll. So it will be very important to get this ball rolling asap for your children.
I would start by contacting the principal in the school you are wishing to enroll with and see where they stand on their numbers for this year and who they would recommend you call next to get the process started.