Here's how I pack my kids for a week at the beach...we stay in hotels and don't do laundry; each kid gets:
- a lot of bathing suits and a cover up
-two pairs of PJ's (they shower in the evening, so I don't think more is needed)
-daily underwear (obvious)
- 3 to 4 outfits that they wear/rotate as needed throughout the week (maybe one gets worn to dinner one night, and then breakfast or an activity the next morning, etc., whatever). Re-wearing is fine because kids shower daily in the evening, and they are only wearing for small-ish chunks of time. My kids are also not particularly sweaty.
-one beachy/pool shoe (like flip flop) and one other shoe (maybe a sneaker)...they wear the latter on the travel day, so it's not in suitcase anyways
I have two kids and this really doesn't take up much luggage.