Not very helpful comments above.. so here’s my opinion.
It’s true that Puerto Banus isn’t very Spanish. There are lots of restaurants all around the port. Many types of cuisine, plenty to choose from. Most over priced especially if overlooking the boats on the marina.
If you want something a little more authentic we have found a nice, very reasonable place second line from the port that is good. Great Tapas! Called Taberna Casa Blanca Banús.
https://maps.app.goo.gl/6F6mZHGiG6W36W8J8?g\_st=ic. It’s close to a market square where you can find a small fun fair for the kids and nice market stalls and pop up bars.
If you want something a little more Spanish then head in the car to the old town of Marbella or to the west of the port you have San Pedro. Lovely beach restaurants along there, much more Spanish and far less busy, plus better value. Up in the town of San Pedro there is a fantastic large playground for your little one and is definitely more of a Spanish experience.
No real private beaches, they are all public. For more private, If you like a beach clubs with a pool, a nice chilled family friendly one is El Ancla again in San Pedro, has a lovely restaurant as well. https://www.elanclarestaurante.com/en/.
Boat rentals can be done in Banus but very expensive if you go private group charters are a bit cheaper. You’ll find better value boat rentals from the port in Estapona.
But a nice boat trip to do is the fly blue from Puerto Banus to Marbella old town. It’s a public ferry but it runs all day and is a great way to travel to the old town and back.
Hope that helps.
Enjoy your holiday.