Ritchie - i've seen one in horizontal rain on the Pembrokeshire coast and it didn't half shake!
my current family tent is a WC Monsoon (old style), 2 adults and two kids job, and after looking at the issue of what happens when the kids get bigger (we've got broadly similar requirements to you) we've decided to go for a two tent solution because a) lightweight, large, durable tents that cost less than £500 are a bit thin on the ground, and b) as the kids get older they aren't going to want to sleep in the tent with us (nor us with them tbh...).
so, two 2 man backbacking tents are the answer. we've got the monsoon, a two man Alpkit Kangri, and when the time comes well go on fleabay and pick up a secondhand Quasar/Voyager/Kangri type thing that the kids can use while they a) decide if they like camping on their own, and b) wreck because they're a pair of clumsy fcukers!
we did look hard for a solution to the 3/4 season, backpackable, 4 man, durable tent problem - but we just kept coming back to that £500+ pricetag, and we decided when the kids want to go camping on their own, we wouldn't be keen on them using a £500+ tent, and that itwouldn't be much use when its not all four of us as it'll be too heavy to backpack.