What kit do you currently use?
I'm working with two camera systems at the moment as I make the move to mirrorless. I still have my beloved Canon EOS-1D X Mark II, which I use with a Canon EF 85mm f/1.4L IS USM, a Canon EF 35mm f/1.4L II USM and a Canon EF 24-70mm f/2.8L II USM lens. Children move fast and the exceptional autofocus and rugged build quality of the EOS-1 D X series is fantastic when you don't know what a session will throw at you – sand, bubble liquid, cuddly toys – with enthusiastic toddlers anything can happen.
In 2020, I added a Canon EOS R5 to my kitbag, which is amazing. I'm really enjoying the face and eye tracking, the vari-angle touchscreen and the 5,940 autofocus positions, which are allowing me to create new and interesting compositions for my clients. I use it with the sublime Canon RF 50mm F1.2L USM, which I like to think of as my desert island lens. I also use the wonderfully versatile Canon RF 35mm F1.8 Macro IS STM, which I particularly love for its macro feature, which makes it the perfect lens for photographing newborns. I'll be adding a Canon RF 85mm F1.2L USM lens to my kitbag in 2021. The quality of the new RF lenses is astonishing.
I always recommend investing in a fast prime lens as it allows you to shoot indoors using natural light, which is a great way to learn about aperture and shutter speeds. The high quality and inexpensive Canon EF 50mm f/1.8 STM is a wonderful prime lens to experiment with. The equivalent for those using Canon's full-frame mirrorless system would be the Canon RF 50mm F1.8 STM lens.