hi davejay, welcome to nissanforums!
frankly, your sentra is quite family-oriented already. i don't know if you checked the rear window ledge, but there are child safety seat tiedowns in every sentra (yes, even my spec v!). i believe there are a total of three mounting positions, not sure if you can use all three at once. also, the rear doors come standard with childproofing interior latch lock-outs, for when your tykes outgrow their child safety seats. my 2003 model also has lap and shoulder belts for all three rear seat passengers, making it ideal for carpooling or for a family outing
also, with gas prices fluctuating for the foreseeable future, having a 1.8 liter four might prove beneficial for your pocketbook. while the qg18de doesn't get as much mileage as some hybrids, with your 5-speed you enjoy 35mpg highway, a nice feature for such a capacious car. speaking of capacious...the trunk is hard to beat, i have yet to fill it with a week's worth of groceries for two adults, and i have that rockford fossgate bass module sitting in there!
so before you consider trading in your sentra, take another look at all it has...and the equity you've got in it. if you want to talk performance upgrades (without impacting any of the features i mentioned to boot), ask chimmike what he did with his gxe, you'll be amazed
good luck on whichever course you decide upon