I'd use a little Old Bay. Also bay leaves, Worstershire Sauce and lemon. For the liquid a mixture of water, cider vinegar and beer or wine works well. If the Old Bay is too spicy, some black pepper corns, paprika, coriander and salt will give a nice flavor. Frankly, the food won't pick up much spice if you go easy on whatever seasoning you use. You could halve the recommended amount of Zatarains and have a pretty good flavor without it being hot. Good luck and let us know how it turns out!
PS, one thing to watch out for is the sausage. I did a boil once and got what I believed to be regular smoked sausage, but it was actually the hot variety. The spice leeched out into the cooking liquid and seasoned the whole thing. I believe it was "Georgia Boy" brand. It still wasn't too hot to be edible for most though.
By the way, I also did a low country boil for the rehearsal dinner for our wedding years back. The guest list included quite a few older folks and a bunch of kids, so I was faced with a similar situation regarding heat. I used only Old Bay as a seasoning and just cut back on it. I think I used maybe a third of a cup for an 8 or 10 gallon pot. The food came out seasoned, but not "spiced". I then left a few dishes of Old Bay on the table for those who wanted more. It worked out perfectly. On a funny side note, many of the older guests were of a certain socioeconomic sector that eats from fine china on linen covered tables. Not snobby people, but pretty well off and not exactly accustomed to "down home" cookery. Everyone was milling about chatting and drinking while waiting for the dinner to be served, and here comes the groom with a huge steamer basket full of shrimp, crab, corn, sausage and potatoes, which I unceremoniously dumped in the middle of a newspaper covered table with a shout of "DINNER'S READY!!". The looks were priceless, but I noticed later there were several of the older "high society folks" standing around the table stuffing their faces without benefit of plates or even utensils. They all raved about the food and fun atmosphere, and still talk about it to this day.
