My run down on the beaches
Abersoch beach is nice but rather overrated imo (ie v crowded considering the other lovely beaches on the Lleyn). The advantages are the easy access into the town which is uber rah (white stuff, crew co etc plus lots of other fancy shops selling organic babygros for £12). There is a cafe and next door deli combo that do nice take away bagettes that are good to take down to the beach).
Footprints cafe (on the beach) does a good hot chcolate.
Llanbedrog beach is much less up itself, but the sand is a bit more shingly. The beach bistro does lovely food and good proper icecreams in a cone. The beach huts are the prettiest imo. Just up from the beach is a big house that is a bit of a gallery and craft centre and also has a tea room. GOOD CAKE LIVES HERE. National Trust car park (if you are a member)
Nefyn Beach is a no frills but good solid sandy beach. I had a jacket spud with beans here once that was satisfactory. Parking a bit of a mare, good 5 min walk from nefyn itself to the beach.
Morfa Nefyn is more of the same (a bit more shingly maybe) but slightly better parking. National Trust Car Park also.
From Morfa Nefyn you can walk round the bay to Porth Dinllaen (or you can access this beach by parking on the headland (£4 iirc) and walking through the golf course. Lovely, lovely tucked away beach and as someone else has mentioned the pub, the Ty Coch. Great to sit with pint and look straight down onto the beach with dc pootling about.
Penllech Beach is utterly divine with hardly anyone there. we accessed it from the costal path, but I believe there is a small car park and then a walk a cross a field (on footpath) to the beach. Big, great sand, no facilities, v cool streams running down onto the beach - perfect for dam building.
Porth Oer (Whistling sands) my favorite . Best beach in the world imho. Good for all beachy things and rockpooling. Cafe does great paninnis with chutney om nom. 5 min walk from car park (and loos) to beach down steep road. National Trust car park.
Aberdaron is a bit more shingly but great views across to bardsey Island. Lots of eating/pub places.
Hells Mouth is worth a visit for a bracing walk and to watch the surfers.
Here endeth my beach roundup. We've not done them all yet
re activities for an 8yo ds not sure if I can help you as we do beach and then cafe and then back to cottage or tent if the weather is foul Pwhelli has a cinema and a leisure centre with pool.
Criccieth castle is fun. Cadwalladers ice cream palour does good sundaes (as recommended by pointydog and confirmed by me)
I apologise most profusely for all the spelling errors of the Welsh names.