On-site properties are so worth it, you may never visit theme parks the same way again. The express pass is just awesome, many will follow with a full explanation. The short version is, with the exception of a few rides in HP, you don't have to worry about crowds. Mid October is hit and miss with Halloween Horror Nights really starting to get ramped up. Again, stay on site and you don't have to worry.
Club level is always a discussion with many viewpoints. You basically get a continental breakfast, snacks throughout the day, all day drinks, and dessert at night. I stayed at Royal Pacific club, and there was a concierge who set up all of our dining plans as well. Here is an example of what you need to consider - there are not many good inexpensive breakfast options. A danish and a coffee is going to cost you $30 for four people, and the breakfast buffets at the resorts (although delicious!) will run you $60-70 pending your tipping style. The experts here can give you other options, this is just an example. Next, bottled water in the park is gonna run $10+ each time unless you are like my family and share the same bottle (ugh). So, if you eat the continental breakfast and take four bottled waters with you when you head to the park, you are saving $40 - $90. Still not the cost of the upcharge for club level.
However, when you factor the value of Express Pass, you will realize that you can eat breakfast, take a short walk to the parks, ride for 3-4 hours, head back to resort, get more drinks at club room, swim a bit, afternoon snack at club room, go back to parks for a few hours, then hit desserts in the evening. Everything is there, maybe just one good meal outside of club room each day (there are so many good meals everywhere). It could be worth it. It really differs from family to family, if you are a family that eats a light breakfast and heads to the parks for 12 hours, then club is prob not the best option. If you plan on being around your resort and taking advantage of express pass, it is so easy to just swing by club room and load up. Very easy to just stop in, grab and go, and could save money.
All that said, my family will probably not choose club level again. We love the Kitchen (Hard Rock Hotel), we all have our favorite snacks at each park, we love the sit downs at City Walk, we love the poolside bar and grill at Hard Rock. We are foodies and we look forward to our annual stay just to eat, and with so many new options opening each year, it is unrealistic for me to think that we won't hit our favorites and try the new stuff. And I can't afford both. Also, I am not sure about your 14 year old, but the 9 year old will likely get a free breakfast buffet with an adult purchase if you ask at check-in. It has always been offered to us, I have an 11 and 8 year old.
Again, if you do choose to stay on-site, don't look at it as two days each park, take Gringott's between the parks and spend the first day to day and a half and hit the major attractions. Spend some time at CityWalk and your resort. Then, the last two days can be repeating your favorites and looking for those special nuggets of great fun (the shows, the fountain, Diagon Alley, Springfield, so much to list). It really is amazing how much FUN you can have at Universal, but still get some relaxation when you stay on-site.
I mentioned that I stayed at Royal Pacific for club. That was a one-night stay in the first week of a 15 day vacation for us. We stayed at Hard Rock for one night the second week and the day we checked out, we never made it to the parks. We rented a cabana by the pool for the day - it may be the funnest day my family has ever experienced. Granted it was mid-July and packed, and I dare not to search it on You Tube - 20 sunburnt, half crocked dads doing an air guitar competition to You Shook Me All Night Long by AC/DC...well, that was about noon. So, we are a Hard Rock family through and through!