I keep a box of old clothes for the children to dress up in

I(主语)+ keep(谓动)+ a box of old clothes(宾语)+ for the children(不定式的逻辑主语)
+ to dress up in(不定式作定语修饰 clothes) .
当然也可把 for the children to dress up in 当作是带逻辑主语的不定式作目的状语.
两种情况,翻译出来的汉语意思都差不多.
句意:我保留了一箱供孩子们乔装打扮穿的旧衣服.(不定式作定语修饰clothes) .
我保留了一箱旧衣服,供孩子们乔装打扮穿.(不定式作目的状语.)

I keep a box / of old clothes / for the children / to dress up in .
the children dress up in the old clothes, the old clothes in the box, I keep a box
I / keep / a box of old clothes 分别是主语谓语宾语，后面是宾语补足语

I keep a box of old clothes是主句
for the children to dress up in 是从句
I是主语keep是谓语
a box of old clothes“一箱衣服”是宾语
for表示“给……”的介词
the children是从句中的主语，to表示目的的介词，dress up in是介词短语

首先是主要成分主谓宾：I keep a box of old clothes.

接下来继续拆：I keep a box of old clothes for the children为孩子准备了一箱旧衣服

最后 I keep a box of old clothes for the children to dress up in 为孩子准备了一箱他们可以穿的...