Welcome to the guarenteed "slightly out of focus shots of our children when using a nifty fifty club"!
You are not alone.
You'll find my posts about the same subject about a month ago!
I have 2 young ones also, and getting them to stay in the frame never mind still, is like herding cats.
My learning points echo the above - use shutter prioirty mode and aim for 1/100 or 1/250 minimum, but be mindful and don't try and take piccies initially less than f7 or f8. This increases the dof so is more forgiving. Bump up the ISO if needs be. As you get better, you can start to increase the aperture.
Centre point focus helps hugely, aiming for the eyes if possible. Back button focussing takes a little while to get used to but helps a lot.
I know it sounds daft but a mounted flash during daylight can also help. The infra red sensor used by the flash to set focus also sharpens everything up.
Shoot in RAW, and adding a sepia filter is really nice.
And finally don't be hard on yourself. I would say my hit rate of shots I keep is less than 1 in 10, maybe even less. I haven't nailed one yet I'm really really proud of, but I'm slowlygetting there, mainly in part thanks to the fantastic advice I've had on this forum.
Best of luck!