They key is to look at the max tow rating of your truck and then compare that to the GVWR (Gross Vehicle Weight Rating) of the travel trailer. To keep give you a little head room I have always been told to shoot for 80% of the maximum.
With kid's your most likely going to want bunks for them, with a Queen bed for yourselves. For my wife and myself the bathroom is a big thing for us, we like the bathrooms with the tubs in them, along with a sink in the restroom. The slide out models give a bit more room, but at a price.
I would start looking on Keystone, Forest River, Jayco, Northwood, Outdoorsmfg websites and look up the GVWR for the floorplans you like. Write those down and then if you have a dealer near by actually take the whole family down and go inside and see how it works for you guys. One of our mistakes was looking online and trusting other peoples reviews without really spending a lot of time on the lots testing them when we purchased our first TT. Everyone will have different opinions on what's the most important.
My brother-in-law is constantly fighting fresh water usage with his two girls, they have a 40 gallon tank and are now hauling an additional 15+ gallons of water. Light weight models usually are 30-40 gallon tanks, while the non-lightweight models go up to 80 gallons.
2009 Ford F350 V10 4.10 FX4 Crew Cab SRW, Timbrens, Leer Topper