do we need advance tickets for Franklin Institute?

I don't think it's necessary, however you might want to just to be safe during holiday seasons / long weekends. If you're visiting from out of town, or just want to visit a whole bunch of Philly attractions and get a discount, you might want to check out the citypass, which is $54 for adults or $37 for kids. It gives you access to the following: The Franklin Institute, Adventure Aquarium, Philadelphia Trolley Works / Big Bus Company, Philadelphia Zoo, National Constitution Center, and The Academy of Natural Sciences, OR Eastern State Penitentiary.
http://www.citypass.com/city/philadelphia.html

It depends -are you an early person-get there when the gates open? then perhaps not. -if you plan to stroll in at lunch time ? I would! - It makes things easier since there is a separate line for families w/ tix already in hand! When we were there (holiday /weekend ) we stood in a line with 3 people ahead of us (tix in hand) OR we could have stood in line with aprox 20 people ahead of us ( no tix in hand) - sure does make things easier! Soooo I say get the tix!
P.S. thanks for the compliment -laurentia99!!

We were there in the fall and walked right in, didn't have to wait at all. It was in the middle of the week in the afternoon.