Hi Just4fun and welcome to GoY. You and they could plant some crocus and tulip bulbs right now and next spring sow some simple hardy annuals. If you want to encourage them to grow vegetables try peas, broad beans and carrots (if you have a light sandy soil). It really depends on the age of your grandchildren just how much they will be able to do but starting them off young is the best way!

Most G.C's sell seed starter packs especially geared towards young children, but the easiest thing is sunflowers, there are lots of different varieties and they could even have ther own little " Tallest Sunflower competition ". Welcome to GOY

Welcome from me to J4F I was going to suggest Sunflowers and Sweetpeas and some of the annuals such as Marigolds, also Peas, lettuce, herbs and even potatoes/strawberries in a container - to encourage the idea of growing edible crops. Cress on blotting paper if they are very young is a good way to show what happens when you sow seeds.