I normally don't recommend buying things, but, here, there are two bits of equipment which will make a world of difference.
First, get a wider-angle lens — 50mm or 35mm or somewhere in that range. The 105mm portrait range is great for isolating individuals, but not so great for "lots of kids", both just because the framing is tight and because the framing will tend towards not very much depth of field. (See What exactly determines depth of field? and Why is depth of field affected by focal length?)
Second, get a flash — something low cost that you can use off-camera. The Godox and Yongnou systems are great budget options, or you can go within Nikon's own system. This is essential because in order to increase depth of field, you'll want to stop down the aperture, which means less light. A flash will give you that, allowing you to stop down without going to slow shutter speeds and high ISOs.
Many people are shy of flash ("I prefer natural light!") because they've had or seen bad results with on-camera flash. Getting the flash off-camera (preferably with a radio-based system — these are very affordable these days) is a world of difference. It does force you to be more aware of light, but that's a good thing for improving your photographs anyway.