And when the container gets too full, you can have the grandkids come along when you take the excess to The Salvation Army store. Build a plywood toy box with a hinged top and paint it bright cheerful colors. Then stock it with toys, games, books and some simple sports equipment, like a badminton set or a softball and bat for your grandchildren to have fun with and look forward to when they are coming for a visit. If you have a nice wide, dead-level yard, a great sports equipment choice is a croquet set. It makes a big hit with grandkids, and you have a fair chance of beating them at it if you practice beforehand.
Few grandparents have problems with obstreperous grandchildren left in their charge.
The principal is, if they love you they will behave themselves to please you, and mostly that works out. But if you're going to have your grandchildren in your charge for any length of time, it's a good idea to establish with their parents the parameters of good behavior well ahead of time. You may find they have quite different ideas on discipline from yours. Spanking, even a swift smack, is a no-no these days. Better be ready and have a strategy lined out.