Always on the go, soccer, travel, school concerts, playing at the beach

The Nex-7 is MUCH smaller, definitely go for it.  The Nex-7 also has some great features like fast rapid fire, twilight mode for great low light shots, built in panorama etc.  

The only reason I think not to consider the Nex-7 is because fewer lenses are available, but it sounds like you'll probably do fine with the included lens, or at most perhaps buy one more.  Here is a list of lenses available for the NEX-7.

nex wud be a better option as going for d5100 after having a d90 doesnt sound to be a good option

u can also consider the CX format cams which are real light and gives good results -

Basing your choice solely on value, and familiarity with the product, the 5100 is the clear winner.  As you stated, you already have a d90, so operating the 5100 is going to be second nature.  Also all of your lenses you already have for the D90 should work on the D5100.

Basing your choice on bulkiness, and potability, the Nex-7 easily is half the size, lighter, and produces among the best photos possible, but is pricey.

I Currently own a NEX-7, and can attest to the quality of the build and images, to be the best I have used, out of over 50 cameras I have owned... but I still see one problem.

If you are going to carry the camera in your purse, neither camera is purse friendly, simply the amount of dust in your purse will likely seep into crevices in the lens, and it wont be long until the sensor and or lens gets dusty to the point where it affects image quality.  

If you are planning to sport it round your neck, I can hands down recommend the NEX-7.

I agree with Foto-Mim. I'd go for Olympus OMD E-M5. Similar price, but overall a better camera. You get weather sealing, IBIS, and better lenses, plus it's faster.

Not much bigger than the NEX, 5-axis in-body stabilisation, weather sealing (good on the beach, with all that sand and splashes), stereo sound out-of-the-box. You can use a Panasonic Lumix G 14mm f/2.5 pancake lens to keep it smaller. You'd better walk into a shop and see them both side by side, hold them in your hands and make the proper decision. Good luck!