Adding a border to your vegetable garden or flowerbed helps create a transition from the rest of the yard to your garden area. The border creates a more finished look, but it doesn't have to be expensive. The right border lets your kids know immediately where the yard ends and the garden area begins, setting the visible boundaries of their play area. Borders can also help prevent your grass from encroaching into your flowerbed.
Use a half-moon edger or spade to create a 90-degree edge where your grass meets your garden bed. This should be a narrow trench marking the line. Dig away some of the garden soil if necessary to create a slight slope to the new edge. The slope of the garden bed combined with a sharp edge creates a natural border that clearly separates the grass from the bed -- at no extra charge. Fill the edge with mulch to restrict the growth of the grass; it can quickly spread into your bed unless you keep the line clearly defined.
Spread mulch around the edge of your garden bed. Many plants benefit from mulch spread around the roots, but to save money, using it just around the edges creates a finished look. This works especially well if your plants are close together where not much soil is visible in between. As an alternative, you can use two different types of mulch -- an inexpensive version to spread around the plants, and a more expensive version just to spread around the edge to make a border. This can be wood chips of a different color from the rest of the garden bed or a totally different type of mulch, such as pea gravel.
Ask a builder for leftover bricks. Builders often have a few bricks leftover from each project, sometimes ones that can't be used because they were custom cut to fit the house. Some builders might give you the bricks, while others might charge you a small amount. The bricks don't have to match to make an attractive border. Mix and match colors and sizes, or dig a trench and lay the bricks at an angle with only the top sections exposed for a more decorative look. This also helps disguise bricks that are shorter than the rest.
Involve your kids in a treasure hunt to find the right objects for your border. Go on a nature walk and look for rocks, or have a contest on the beach to see who can find the most shells. Broken or whole, shells can make a creative and effective border. It might take several nature walks to find enough rocks to line your garden bed, but there's little to no cost involved.