can i use playground pebbles for my hydroponic garden.

yes. Any mediums can be used for example- bricks, stones etc. as long as they can support the plants.. The most important thing for the plants is the nutrients in the liquids that you put at the roots of the plants.

Around here, those pebbles would most likely be crushed limestone. Some plants really don't care for all the lime that leaches from the stones. I tried using it for a couple of orchids. The orchids I put in lava rock thrived, the ones in the limestone croaked.