I won't rehash the reviews- it's an overall great beach that's still a hidden gem compared to Ocean City or the NJ Shore. We went for a week in July and it was never overly crowded, and was clean and very family friendly. So if you've already decided to go with some young kids, here are some tips- some Rehoboth specific and some not:
Parking- we went at the height of the summer season (july) and a quick trick we learned is that the parking meters on Rehoboth Ave (main road leading to boardwalk) don't start until 10am and you can prepay them. In other words you can arrive at 8am (which for those of us with young kids is called a "late start") choose a parking spot literally 50 feet from the beach, feed $4.50 into the meter ($1.50/hour with max of 3 hours) and your spot is paid for until 1pm! You've just gotten 2 free hours of parking AND a super close spot. For those tech savvy parents we did the parking app and it worked fine (you can feed the meter from your smartphone) but do be aware that there is a $0.50 extra fee PER TRANSACTION so if you are going to use it- do a lot of hours at a time or else you'll be paying a lot extra.
Umbrellas- there are umbrellas for rent- when we went it was $12/day + tip (some of the people do linger around after you paid them $12 to hint for the tip...). If you are going multiple days and have young kids (like us) You can get inexpensive beach tents off Amazon for $50 which you can use repeatedly year after year and give better protection than the umbrellas. (Even in one trip, three days of renting the umbrella is already $14-15/day x 3!)
Nearby Henlopen Park has FREE (yes FREE) bike borrowing, even has bike trailers and tandem bikes to tow your little ones in. Do get there early, when we went there was already a line at 9am when it opened and there are only 2 trailers and 2 tandem bikes. At the Nature Center- the Nature center itself is small but have some tanks of fish and a petting aquarium with 2 sad looking horseshoe crabs. (Yes I know this isn't Rehoboth Beach technically, but I don't feel like writing a whole new review for Cape Henlopen Park!).
Note there are no changing rooms or bathhouses so be aware. Come dressed for the beach! There are public restrooms but technically you are not allowed to change in them.
I would also recommend, depending on how long your vacation is- to really consider renting a house further away instead of splurging on a beachside suite hotel or nearby house- given how easy the parking is (see above) we spent CONSIDERABLY less by renting a very large house (3 bedrooms bedrooms and kitchen- 2800 sq ft!) that was "far away". Plus we saved $$ by being able to cook and store food for packed lunched as opposed to having to eat out all the time. So we chose a place that was 15 minutes drive away- got rock bottom prices but still was able to park steps away from the beach and on non-beach days could easily go to Outlets, grocery shopping and other attractions. Plus the development we stayed in had a pool!
The only reason I subtracted 1 star was that there was one rainy day and there is very little to do for the young-crowd indoors. No really fun museums, no indoor activities- a movie theatre and really that's it. The boardwalk is also, admittedly not as extensive or grand as those in NJ or Ocean City, but it is simply and fun enough for the younger crowd.