E.H.
I've been taking my kids to the beach as early as 8 days old. My husband and I surf so we go to the beach a lot. I've never heard of baby powder getting sand off... Sand will easlily brush off dry skin. Its when your skin is wet that it sticks. Though dumping water on it also takes much of it off. My biggest tip is to accept that you will have sand everywhere. If you have little, little ones, keep a bottle of water close to pour on your hands or their face if they get sand in their eyes and keep one towel out of the sand (hang it on an umbrella or something if possible) to dry hands and faces. Teach older kids to "gum" food instead of touching teeth when chewing to cut down on the grit between teeth. Throw everyone under a shower on the way out. If there is no shower, take a big jug of water, leave it in the car (gets warm) or outside the hotel/condo to wash the sand off. Always put sunscreen on before getting there...otherwise you end up rubbing sand all over your kids. Wetsuits are great for keeping sand out of sensitive areas if you are going to cold water beaches...but honestly I don't think sand bothers kids too much, only when they get chaffing, which if they do, put some vasaline on it the next couple of days (it will attract sand, but won't add to the chaffing), or you could get some body glide anti chaffing stuff. A dunk in the ocean will get you back to square one, you just have to keep kids off the sand until they dry...a large beach blanket is good for this...you just may need to shake it off MANY times during the day.
Water tips...if you get caught in a rip tide (meaning you are swimming or floating and are getting pushed out too far) swim parallel to the beach before trying to swim back in, otherwise you are swimming against a current and will tire out quickly. You can always ask lifeguards (if they are there) where rip tides/currents are that day. When playing in the waves, if a big one is coming and you don't want to get clobbered...dive under it, or if it is shallow enough (and the wave isn't too big), jump up and into it.
Have fun!!!