Maine, of course, since you posted on the Maine forum.
My kids are 5 years apart and when they were younger we spent a lot of time up there.
Here's a few places that come to mind. Just remember you are going during prime tourist season so all these spots are best fished at first light (for better fishing and parking) so I hope your grandkids are early risers:
1. Kennebunk river mouth in Kennebunkport - east side, there is a parking lot and jetty, best fished with a moving tide.
2. Parsons Beach, Off Parsons beach Rd near Kennebunkport - open ocean with creek mouth, best fished during a running tide, waders are helpful but stripers do sip sand eels close to shore in the AM within reach of a spinning rod. Don't be afraid to try the little bridge on Parsons beach Rd during a running tide.
3. Wells Beach pier, pay for the beach parking, get there early or it will fill. You can catch small stripers all day on either side of the jetty, but the right side (ocean side) of the jetty can be good just fishing the wash beneath your feet all the way out. Family can hang on the beach while you fish all day.
4. Marginal Way - Park at Perkins Cove and walk the path - first light with a high tide is best, you can catch stripers and small Pollock in the nooks and crannies or cast off the rock outcrops for bigger fish.
As far as equipment, don't go big, regular freshwater spinning tackle with 8ib test that they can cast farther is just fine as most of the fish are small and medium. 1/4 oz to 3/8 oz jig heads with fluke or paddle style bodies work well. Northland mimic minnows silver/black work well. You don't need to go crazy with lures. If you want to use bait, try sandworms on a hook with sinker.
Just a few words of warning:
Jetties- you do need to keep an eye on the kids but find a place that has a good platform
Bugs - Marginal way and Parsons will eat you alive with skeeters and other bad bugs at first light, bring repellent.
Parking - first come first serve unless somebody can drop you off.
Have fun!