I strongly recommend staying at Fern's place - it is clean, comfortable and there's more than enough space for a family of four (including two lads aged 20 and 16 in our case).
While we didn't spend very much time in the apartment (as there's so much to see and do in Seattle and we had a limited time), the lounge area provided a good space to relax at the end of the day.
The bedrooms are a good size and with an ensuite bathroom in each of them plus an additional one, it meant that there weren't any queues to use the shower. The beds are very comfy and there is plenty of storage space.
There is a fridge in the lounge that meant we were able to pop to the nearby supermarket and get some breakfast stuff for the following morning.
Parking is available on the driveway, and the apartment itself is easy to find if you're driving. There's a bus stop just round the corner if you're reliant on public transport although I don't know how long it would take to get into town.
On Loren's recommendation, we went to Lake City (about 20 mins walk) for a drink and bite to eat at Elliott Bay Public House and Brewery - it's not in the smartest part of Seattle, but the atmosphere in the bar was great.
Finally, Fern was very helpful in providing loads of information before we booked and then again before we arrived to help us plan our stay in Seattle.