Righto Aquasea, as far as the northern beaches goes I think you need to stick to the lower end (the southern end, nearest to the city) and that way your DH can commute in either by the Manly ferry or by bus/car. One major disadvantage of n. beaches is terrible public transport, no train service etc. So you need to stay within 15-20 mins of Manly I'd say. That means (in order of how the beaches go up the coast from Manly and away from the city - Manly is closest - Queenscliff, Harbord, Curl Curl. Lots more lovely areas beyond here but I think it would be too much of a headache commute-wise. The whole of the n. beaches is very family friendly TBH so I don't think you'd have any worries there, though I only properly know about the top (northern) end of the n. beaches.
Those are the suburbs that are actually on the beach.
Then there's an area called Middle Harbour (get your hands on a map so all this makes more sense if you can) which is absolutely beautiful, lots of sheltered waterways and protected coves and beaches. Seaforth is very nice and so is Balmoral - overlooking inland waterways, very tranquil, lots of families, but also quite posh so you'd need a nice big budget for there .
If you can persuade DH further up the n. beaches Collaroy and Narabeen are worth considering. Quieter, but lots of cafes, very surfy lifestyle, lots of families. Basically the further north you go the quieter and more green it all gets, though wherever you are in this area you're so near a beach or water you never feel too hemmed in or surrounded by suburbia.
I hope this isn't too bamboozling and that it's a bit of a help. Don't rule Manly out, by the way, there are plenty of little villagey enclaves within Manly itself such as Manly Cove and Shelley Beach, which are family friendly with sheltered beaches and away from the hubub of the Corso.
One final suggestion completely out of n. beaches would be Watsons Bay, which sort of looks back at Manly from the south. Beautiful sheltered beaches, lots of cafes, direct jetcat access to city (15 mins?) and gorgeous houses. Could be a bit of a budget buster but thought I'd throw that one in there...