You're going to moor at Town Pier downtown. Any of the beaches are too far to walk from there. Probably what you want is a resort with a beach and on-site dive operator - depending on your arrival/time in port you could do boat dives also.
There's a few beaches on Bonaire (about 1/2 dozen) but literally there's
no facilities at any of them and very little shade. Except the kitesurfing beach at Lac Bay which is too shallow to dive.
Dive Friends has a shop a few blocks north of the pier (too far to walk). There's a couple of paid guides on Bonaire - VIP Diving or BasDiving - you might see if they have other options. Another option could be dive at one of the resorts with a beach. There's a couple of those.
One decent option could be the Divi Flamingo. It's just south of the pier - actually an easy walk. They have a small beach - restaurants, a small casino etc.
Divi Flamingo Beach Resort - Attractions Near Divi Flamingo Beach Resort And Casino The brown stuff in the water could be ironshore or coral rubble though so at least surf shoes are required for the beach-goers.
IDK their walk-in arrangements but we've eaten there w/o any fuss. They have their own dive operator (Divi Dive) on-site - depending on your arrival it should be possible to get out on a boat - at least their single tank afternoon dive if you wanted. Book that in advance with them - most shops on Bonaire have sign-up boards so they'll add your names if you call. There's a decent shore dive there also. The Divi is currently undergoing renovations and one of the things mentioned to be improved was the beach. As of last month it still hadn't been according to another poster.
Diving Flamingo - Bonaire Dive Resort - Diving
A little farther south - too far to walk - is the Plaza Resort. They have a nice beach but IDK their walk-in policy since it's gone AI. Maybe they have a day use rate. There's a beach in front of Coconut Crash bar that's usually pretty busy (volleyball etc.) Their shore dive is 18Palms on the north end but their boat operator is Toucan Dive on the south - it's about a 10min carry. At least they stage tanks there. Toucan had nice rental gear last time I noticed.
https://www.plazaresortbonaire.com/en The parents/kid would find enough to do.
Just north of town - too far to walk - Den Laman/Sand Dollar condos share a small beach - both have dive operators on-site. Bonaire Dive/Adventure is there also - we've rented from them on a daily basis.
Bonaire Dive and Adventure Dive Friends is also at Sand Dollar but their boats are kept at their Yellow Sub location on the north end of town.
Dive Friends Bonaire - Best Bonaire Scuba Diving and Services
The beach is on Bari Reef - one of the better known dives on Bonaire. There is a restaurant on each property and a couple others nearby also.
http://www.bonairepros.com/images/uploads/accommodations/3 x 2 Den Laman 2-1.jpg The area where those two cars are parked is just north of an area supposedly being transformed into a public concession beach this year but I don't know the current status. That's definitely cab/car rental distance.
Arguably the other nice beach/resort on Bonaire is Harbor Village - the nicest on Bonaire. But their shore dive is meh at best, a small wreck and coral rubble deposited from the last hurricane. It's pretty upscale and IDK if they do day rates.
Unlike Cayman there's maybe a dozen cabs on Bonaire. I think you can pre-book one here:
http://www.bonairetaxi.net/
hth, If someone suggests Buddy Dive - a really popular dive resort - they have sand - on top of 15' ironshore cliffs down to the water. So not exactly kid-friendly beach play.
