Before looking for a travel agent I'd start by deciding what you want to experience, and how much time you'd have. The one-way cruises (which you'd need) to and from Alaska all start or end in Vancouver BC; all the other cruises (from Seattle or a few from San Francisco) are round-trips that won't allow time for any and-based touring, due to US maritime law.
Travel agents, who work on commissions, will gladly set you up with cruise-rail-hotel packages that will maximize their revenues, not necessarily your enjoyment. You have a year to plan, so use at least part of that year to familiarize yourselves with the cruise and land travel options available. As mentioned, 2022 is likely to be a very high-demand year, with higher prices and possibly less selection in cruise options available, due to all the pent-up demand.
Personally, I would strongly suggest looking at self-drive rather than rail touring options. The Alaska Railroad's route system is very limited and - just my view - wouldn't be ideal for families with kids. With a vehicle you can travel off the beaten path (a little at least) and stop or detour as conditions warrant or as opportunities arise.
Also look at the details, which as you know sometimes can harbor devils. For example, if you want to visit Denali National Park, how will your 5-year old handle the 8-hour round trip ride on a converted school bus traveling from the Denali Park bus station to the main visitor center inside the park? Or if you plan to tour the Kenai Fjords, how does your group handle open ocean in a small boat? Knowledge is power, and in the case of Alaska, the more knowledge the better.
I would look at
Cruise Critic for (endless) advice on the cruise options - cruise lines, itineraries, etc. - and at the
TripAdvisor Alaska board for a start; there's lots of expertise to be had there.
Hit the books.