If you are looking for a fun, family-friendly place in Hiroshima to have some outdoor fun, the Senda-machi park is a fantastic resource in Hiroshima city. It is a fantastic place to visit anytime of year for outdoor activity and fun, but is especially beautiful in Spring when the Sakura trees are in bloom and people gather under the pink blossoms for Hanami picnic-ing.
For smaller kids there are playgrounds to enjoy, two long roller slides and a circuit of stairs and mazes to run and climb over. Net ropes to navigate and other interesting places to play. For older kids, there are areas to ride skateboards (no ramps or courses though) and roller skates. There is a traffic safe running, skating, cycling circuit which is a great way for kids to practice new skills.
In the middle of the park, there are a line of trees on a grassy slope where parents can settle with stuff and everyone can join up at for lunch or breaks.
On the side of the park, there is a small ice-cream shop which also sells some drinks. On the other side of the park (follow the path out on the right side of the large apartment block, next to the library) there is a fantastic supermarket "Fresta". Here you can get fresh fruits, breads, readymade rice balls and Japanese bento boxes full of good food as well as readymade sandwiches and other dishes. This is also a useful spot if you are in need of boxes of juice, water, wet-wipes, tissue, diapers, sun-cream and other family essentials.
At the far end (sports center side of the park), there are tennis backboards and a "trim-trail" set of climbing equipment for sit-ups, pull ups and that sort of fitness (adult sized). There are public toilets on this side and on the playground side.
There is a big field in the middle of this park where groups come to play baseball or soccer on most days after school and on the weekends. When teams are not using it, you will see families playing ball sports or stomp rocket or learning to ride bicycles and other activities on this field.