An apartment is great and I do recommend them -- BUT -- remember you will NOT have the services you associate with a hotel. In most cases there is no daily change of linens(usually once a week on a weekly rental), you are expected to keep the place clean/tidy, etc. It is rather rare to find a Paris apartment with two bathrooms (with 5 people one bathroom could be a major roadblock!)
There are some very nice apts. out there, but they cost quite a bit (i.e. Parisperfect for example - which is a good company, but expensive). In general, you get what you pay for in apts. If it seems to be too good to be true, it probably is. Photos do lie 
I'm assuming some of your family are kids, and if they are used to a certain degree of luxury at home they might find a Paris apt. lacking unless you go for "high end".
Personally, I'd say get two hotel rooms. You are assured of 2 bathrooms for one thing. Yes, some apts do have 2 but you do have to search them out (and in some the second bathroom is really just a w.c. & small sink).
If you go with an apt. check ALL the details in the contract and be sure you understand the "fine print". All require a hefty payment upfront. They advantage with a hotel is that you do not pay till you check out. If for any reason you need to cancel the apt. you need to be aware if you can get your money back (or a portion of it).
It's fun to have an apt. and shop for some grocery items, etc. but that is not for everyone.
