My kids are older than yours (10&8) and it was tough as hell in the early days - I found the mental load much much harder.
Now they're in the "golden years", I find it much easier to balance but, like you, we have an amazing helper who deals with school runs, after school activities, cleaning, dog walking (when needed), babysitting, house sitting, errand running and homework (to a certain extent).
I'm very lucky that work, school, home are all very local (live on a small island), so don't have the challenges of long commutes. I travel for business occasionally and it's no longer a juggle with kids being much more independent.
Personally, I found that the older the children are, the less mental load baggage there has been on me, but I may look to scale back in the next few years so I can be around more as they hit high school.
It is nigh on impossible to reach an acceptable work/life balance when working in a FT stressful role. The only thing I found was to throw money at it, and of course that's not an option for many.
I'm very strict about the time I spend working for the most part. I'm in my office at 7.30am straight after school drop off, and I work through lunch most days. I usually head home around 4-4.30pm and start dinner, but will generally jump back online after that or after the kids have gone to bed.