I can't recommend the IC over the FS because I love the all villa design and indoor/outdoor room layout with private pool at FS and having in-villa poolside BBQs, and I haven't stayed in the villa categories at the IC yet.
If the IC's design intrigues you, you could spend one night at the IC before going to the FS. The distance is less than an hour by driving.
I believe older kids would find the IC more interesting regardless of the room type because it's built into the mountainside. You ride buggies around and use a funicular to travel up/down the different elevation levels (they call them heaven, earth, etc) of the resort. There's more nature and the design is whimsical.
IMHO Hoi An town is extremely overrated and mostly just exists for the photo ops and pretty paper lanterns.