Depending on which position your DH has within the company, he might have a parking spot included, especially if a company car is part of his allowance.
There is also what is called Early Bird parking that will offer a flat fee under certain conditions which will vary from one parking to the other (enter before 9.30 and leave after 3.30 for example) . Wilson and Secure are the main chains that cover 90% of the CBD parkings. Every other building has a carpark.
For the Northern Beaches, there are no train here. Manly ferry or bus.
If you live on the Northern Beaches (NB) , you get a parking sticker that allows you to park for 12 hours on the beaches, if you don't live in the NB, you can purchase the sticker for a few hundred $.
If the office will be in one of those corporate hubs like Super Centre Belrose or Frenchs Forest Rodborough Rd, he will need a car.
Access to the NB will imply a bridge, the Roseville bridge or the Spit bridge, unless you go high high and do Mona Vale Road via St Yves. The bridges at peak hour will be slow, especially the Spit.
However, this is not very relevant. You do not pick a suburb for ease of commute for your DH. Pick a suburb you and kids will like and thrive. Australia is big, Sydney is big, a long commute is part of the package. Of course, nothing crazy, like living in Central Coast or Palm beach, but if unsure between two suburbs, pick one that will be easy on you and kids. Can the kids walk to school or school bus, or do you need to drive them to bus stop, because many NB's suburbs are like an octopus. They have these arms extending to nowhere, and unless you have a car, you are dead, especially because the NB are not flat. Plenty of very steep roads, around Frenchs Forest, Beacon Hill, Forestville. Do you need to grab the car to buy milk .? The beaches suburbs like Manly, Curl Curl, Dee Why, Narrabeen are flat so it is nice to move around with a bicycle. Freshwater is not flat, but doable. This said, you do not see many bikes around compared to Europe where I come from. There is a car culture here. Shop is 10 min walk, people will use the car.
Some suburbs are luxury dormitories. There is nothing to do after work. The sole café closes at 3pm, there might be a chicken shop doing takeaways, and that's it. Davidson is like that. Some areas in Frenchs Forest as well. Streets filled with mansions and not a single soul around.
I come from one of the most densely populated place in the world (Monaco) and sometimes for playdates, dinners or book club, I drive to these ghost suburbs, empty, silent, and the only cars are uber eats drivers doing a delivery.
Start in an airBnB, then visit the suburbs on your list at certain times of day.