I was taking photos of my friends 3 year old daughter playing around with my 2 year old son and she just stopped whenever the camera was near her or pointing at her - especially the 50-140 f2.8. With the flippy screen tilted up and the Electronic shutter activated on my X-T2, I held it as waist height looking down as if I was fiddling with the camera settings. Click, click, click and she was none the wiser. When I was further away I went back to mechanical shutter, CH on boost mode for 11fps to freeze faster motion. I got some absolute belters using this method for her parents who are good friends of ours. If your camera has a flippy screen - try it, you'll be surprised.
Another tip, avoid eye contact because she'll know you are up to something. Occasionally point the camera away or up at the sky in between shots.