Lens question

I'm assuming your 50 is an f1.4 or f1.8. If so, and you're shooting at that aperture, you're going to have quite a narrow depth of field - meaning that the space font-to-back of your scene that is in focus will not be very large. If the three kids aren't all uniformly beside each other, ones forwards or backwards fro the point you're focusing on are going to be out of focus. First thing to do is to stop down the lens a bit, maybe to f4 or so. This will let less light in, so you'll have to drop your shutter speed or raise your ISO to compensate, but you'll get more depth of field, and so more of the kids will be in focus. If that's still not enough, change the aperture to f5.6 or 6.3 - basically, the higher the f number, the smaller the aperture (the "light hole of the lens"), the bigger the depth of field. So play around with it until you get all you want into focus. Or be creative with the narrow depth of field of larger apertures, and make the out of focus bits a feature of the shot. Use A mode (Aperture Priority) on your camera to manually set the aperture, and let the camera figure out the rest of the settings for you. Experiment with different aperture settings.

I personally prefer the 24mm focal length to 50mm for family shots, so that other lens could be an option too. Use your kit lens at 24mm to see if you like that focal length (obviously it won't be f2.8, and won't be as sharp as the prime, but it will give you a rough preview of what you'll get in the frame). I assume the lens you're talking about is an f2.8, so it's still pretty "fast" in terms of aperture, but as it's a wider lens (in terms of focal length), it will naturally have a better depth of field. So at f2.8, it will have more depth of field than your 50mm at the same aperture. So with that lens, you might have more leeway with depth of field than the 50mm. But that's not to say you can't get great, in focus, family shots with the 50 once you use the appropriate settings.

Just remember that just because a lens is 1.4 or 1.8 or 2.8, it doesn't mean that you have to use that aperture. Change the aperture to suit the effect you want to achieve. In fact, most lenses tend to be a bit sharper stopped down a bit to a lower aperture. Only use the full, widest aperture if you a) are in low light and want to absolutely maximise the amount of light you can gather, or b) want to creatively use the narrow depth fo field.

I have often fallen into the trap of buying new/better kit to get better photos. A bit like a golf player constantly upgrading his clubs.

Sometimes its better to spend some money on training rather than kit. A good photography course is cheaper than a lens and will always be of use, no matter what kit you get in the future.

just have a play with this to give you an understanding of the DoF etc.

http://www.canonoutsideofauto.ca/play/