Traveling in a motorhome is great, especially with kids. The pros compared to a trailer or 5th wheel, are being able to get up and use the bathroom or get a drink or snack without stopping. Well, probably not a good idea for the driver to do that, but others can! And camp set up is a snap. Pull in, hit the levelers, and hookup utilities. Overnight stops are a breeze. The cons...well, I really miss my truck! I have a gas MH, so pulling a full size truck would be too heavy, so I now have a smaller vehicle to pull. Also, the motorhome has significantly higher insurance and maintenance costs compared to a trailer. And I recommend have a vehicle you can pull behind (a toad), so you don't have to break camp and drive the motorhome to sightsee. There is additional cost in setting that up, and only certain vehicles can be pulled four wheels down.
We have a gas class A. I would have loved a diesel, but upfront costs were a lot more, maintenance is apparently a lot more...you get the idea. The diesel was a lot more money! For me, the gasser gives us everything we really wanted, and we could afford it (barely!). But diesels give you more power, so if you plan on pulling a truck with a golf cart in the back, you may be able to do that with a diesel. Most gassers are limited to towing 5000 lbs.
As for length, ours is 35'. It works for us. It is maneuverable, yet gives us enough space and storage. What length works for you depends most I think on floorplan. Look at lots of units (you can start online), and focus on floorplans that you think would work for your family. I think the majority of gas A's are in the 32-35' range. Diesels start from there and go up to 45' I think. If I had a diesel I'd want to limit it to 40', just to not be too limited on what campgrounds I could fit in.
Finally, I'd go for the booth dinette. My kids love sitting at the dinette while traveling to color and write. I'm not sure, but I don't think you can sit at a table and chairs in that way...as I don't think they are secured. I could be wrong about that!
Have fun shopping! Lots of deals out there right now.