Yes, this violates standard gardening principles. However, if you do this, and put your plants close together, no one can tell if a few weeds are present. Some weeds have nice flowers too. Planting from seed can be tricky. It is easy to forget what you have planted.
Sunflowers are an exception.
They are large, dramatic and easy to grow. They are great to plant over things that have died or have just finished blooming for the season. Make the garden a family task. If you have the type of garden that requires less weeding and fussing, the kids are more likely to be enthusiastic. If the kids are to help, avoid some of the more toxic sprays and herbicides.