I remember when I first started taking photos of my nieces and I bought a lens that was sharp but the focus was so slow I missed every single smile. You really gotta be careful with some older lenses because if the autofocus cant keep up with a toddler, the sharpness doesnt even matter. Tbh I would suggest the Sony FE 85mm f/1.8 for your budget. It has been in my kit for years and it’s super reliable. It gives you that nice blurry background but the focus is snappy enough for kids. Just make sure to double check you arent buying the crop sensor version if you have a full frame camera. Bought the wrong mount once myself, so just watch out for that. It’s easy to use, though you might want to consider that 85mm is kinda zoomed in so you will need some space to work.