your work group sounds like a good template for a variety of projects--i've seen these work well for writing groups, too.
may i just add another preliminary step? may sound old-fashioned, but will save you time and money and headaches later: get a bunch of dwelling magazines that focus on landscape design, go through them with your family, everyone freely commenting and cutting out the photos that appeal to them. doesn't have to be your climate or budget or region, but get all the pictures you each like into a pile.
then go through the pile together , on what catches the eye, or calls to your esthetic sense about each one. make notes on a sticky to attach to the back of the photo. no negative comments or money issues on this first round, and let everyone's comments be treated equally. next split the pile randomly into groups of 4 or 5 pictures, and have everyone vote, one group of pictures at a time, rating them favorite to least favorite. take all the favorites and discuss them freely. what aspects are viable and adaptable to your project? do the same with the other piles.
if someone feels very strongly about a certain detail, make a note of that. gardens tap into deep places we cannot always understand or explain. we have found these one-off ideas can reveal an organizing principle for a project, that ultimately brings peace to the whole process.
chances are, someone in the group will be happy to put all this into a notebook--let the most enthusiastic handle it, no matter how young. this will be an important reference for you as you proceed. when you're working with the professional, their vision and esthetic will necessarily creep into the planning. you want to end up with a yard and garden your family can feel is their own, so be sure that the key aspects of the favorite photos are retained as the project progresses.
these ideas come from our own garden design projects. currently, we have 2/3 acre of ornamental plants, with paths, berms, stone walls, vegetables, trees, and on and on. my husband was eager to wield the shovel, but i insisted on this thought process, and we've been enjoying the benefits for many years.