。
She called the children together to her side.
Literal
She [topic-は] children [object-を] beside-her called-and-gathered.
呼び集める ('to call together, summon') is a compound verb stitched from 呼ぶ ('to call') and 集める ('to gather') — Japanese forms many such two-verb compounds where the first verb's masu-stem fuses with a second verb to specify manner or result. Other examples: 呼び出す ('call out, summon'), 呼び戻す ('call back'), 呼びかける ('call out to'). そばに ('to (one's) side, near, beside') is the destination of the calling-and-gathering motion, marked by the directional に.