By safe, I am guessing you mean you do not want pesticides or fertilizers in the mix.  By its very nature, soil is organic.  There are options available at garden centers that will be labeled Organic but you must read the labels for ingredients.  Some will be labeled for containers and some as potting soils.  We do not recommend specific brands.  The personnel at your locally owned garden center should also be able to guide you in the right direction.  Congratulations on including your grandchildren.  It is a wonderful joint experience they will remember.