Lots of kids are not into it. In the general population, very few are naturally comfortable in front of the camera. A common method is to shoot often enough that you are ignored. This works pretty well if you are the parent but does not work as well if you are working because you don’t have the time required to become part of the furniture. If you are shooting kids while they are running around, you will want a medium aperture for depth of field and a fast shutter speed to freeze motion, so extra light is a help. Speedlites have a very brief flash duration so even with moderate shutter speed, they pulse of light will stop motion. High Speed Sync is useful for fill light, setting a slower (sync speed or below) shutter speed and using a single flash pop is better for stopping motion.
The best thing about a pop-up flash in newer bodies is that it can control an off camera flash. To a degree it can even control studio strobes if they are set up for it. The worst thing about a pop-up flash is that it is perfectly positioned to inflict red eye upon your photos. There are a few gimmicks that are offered to help tame a pop-up flash, but none are that wonderful and you are better off saving up for a Speedlite. There are third party flashes available at usually for less money than the Canon lights, I have not tried any of the current crop. I have old third party flashes from the film days and was never very impressed by them. I was really impressed when I put a Canon 430 EX II on my hot shoe. It delivered excellent results. I also have a 580 EX II which is similar but a little larger and it can control the 430 using flashes of light. The best arrangement is the 600 EX RT which has a radio built in. This lets you use a ST-E3-RT transmitter to control up to 15 lamps in up to 5 groups, and you can adjust power to each group from the camera. You can also put a 600 EX RT in the hot shoe and use it to control other flashes, there is a lot of flexibility. The drawback is the cost. My collection of 600 EX RT’s cost the same as my studio flashes. But they are easier to take on location because they are smaller and lighter. Nikon make nice stand and umbrella kits that come with a carry bag. Wescott have softbox kits with stands, you can get round or rectangular softboxes so you can choose the catch light shape you desire. You can get little softboxes, grids, snoots and bounce sheets that work with the flash on or off camera. There is a lot of light modification gear available now.
Outdoors, or if there is good window light, you might benefit from a reflector or two. You can use white foam core from your local craft shop. You can frequently get it for a couple of dollars per sheet, which is a lot less than a dedicated reflector will cost. Dedicated reflectors are more durable and usually do some other tricks, but at 20 to 60 times the cost, you can throw away a lot of foam core before you get to the expense of a dedicated reflector.
Here are a couple of videos you might like: