Someone else mentioned South Padre Island. Native Texan my whole life and I had never been South of Galveston prior to COVID and have been twice. We love it.
There is one all-inclusive resort and its in Florida called Club Med. It is NOT fancy but it is a lot of fun for the kids. They are sports focused, thus they always have lots of activities. It has a beach front - but it is more like a lake, it is brackish water. They have kayaks, paddle boards, sail boats and more. They have volleyball, basketball, pickle ball, mini-golf, flying trapeze, golf lessons, etc. It is all included (note regular golf is not). They have one main buffet restaurant (which is waiter service right now), a snack bar open during the day, and a nicer restaurant at night that requires reservations (I think it is adult only). Shockingly their buffet is really good and a nice variety. The rooms could def use some upgrades; some are nicer than others depending what section you are in. The family rooms are nice because its bedroom (no door), then bathroom, then a front bedroom that has twin beds. All the rooms are spacious. 3-4 pools, one is adults only. They have nightly entertainment (well some things may change because of COVID). Bar is bar brands only unless you pay extra.
Disney/Universal is a ghost town right now. Rides have 5-30 minute waits (most from what I am hearing in the 10-15 minute range). Prices are significantly going up next year thus it is a good time to go IF rides are your thing. The typical Disney magic isn't there due to masking, no characters, etc.
The JW Marriott in San Antonio is another good option if you want to stay close by. Lots of stuff to do in SA.