Your question is very general and wide ranging. It's really impossible to answer on a forum like this. People would be writing treatises. The internet is full of free information. Decide what you'd like to grow and research specifics. Find out whether it is feasible in your climate and whether the conditions in your garden are suitable. Find out where a garden should be placed, how to prepare the ground and how to maintain it. Then get the seed from a shop or online. Packets of seed for common plants cost very little and it is far more likely you'll be successful with appropriately chosen types from proper producers than by getting a random selection from unknown sources. If you don't want your children to be disappoint do your research and start with good materials. Here's a start https://messymotherhood.com/beginners-guide-gardening-with-kids/