It is always safest to maintain valid nonimmigrant status independent of your pending I-485 if possible. You can file the L-1 extension 6 months in advance of your expiration.
You clearly want to file for the extension well enough in advance of the expiration date so that there is no gap in your and your family's authorized period of stay. Make sure that whoever prepares the renewal paper work covers your L-1A *and* your family L-2. It is a separate form for the L-2. If you are a new office L-1A, make sure that you have met all of the requirements for a new office extension. Review this with the lawyer well in advance of expiration (6 months before preferably). If it is a new office petition and you intend to apply for the EB-1 green card make sure you apply for the green card after the L-1A two-year extension has been granted. Best of luck.