Hi, Well if this is in front of a stable, you can't go wrong with composted horse manure. I have horses and use it almost exclusively. Just be sure the manure is aged. I add this in the fall or early spring so it breaks down even more by planting time. In addition (or by itself) you can also apply a slow-release granular fertilizer in the spring. it will feed the garden all summer long and won't harm the kids. As far as mulch goes, I'd look at what's available in quanity locally. Personally, I like shredded bark because it's easy to walk on and lasts a year or two before needing to be replaced. If you have to add soil, I'm sure you can find someone locally who sells topsoil.