这是个定语从句,where和in which一般是可以互换,
where 不能引导这个句子,where是关系副词吧,to也是副词, which是关系代词 可以,代指前面的名词或者事件
这句子应该这么看, the kid can play in the garden 不是play the garden 吧,
介词in 一定要带上, 所以是in which which 代指前面的 grden
there is a garden where the children can play.=there is a garden in which the children can play.