Great project - Grow carrots in your garden - in a partially submerged 2 liter soda bottle with the bottom cut off, enabling the kids to watch the roots & tops grow at the same time. and to see the carrots develop.
Not with concrete, but maybe clay? Get the children involved with these cement stepping stones! All you need is some cereal boxes and quick drying cement. Decorate with glass stones. Excellent idea for the kids to get their garden creativity flowing!
Use toilet paper rolls to start your plants. When ready to plant, stick the whole roll in the ground. Roll will decompose.the toilet paper rolls got moldy before the plants grew. I will stick with growing my seeds in plastic containers .