"Zhang Dong made a lot of toys for my brother", I think in English you would also say "Zhang Dong made my brother a lot of toys". It is just a different focus of connotation. It sounds to me that the first one "张东做过很多玩具给我弟弟" does not end here and there is more information would be delivered after this phrase. "张东做过很多玩具给我弟弟, ......" The focus of this sentence is 张东, you might want to talk something more about 张东. The second sentence "张东给我弟弟做过很多玩具" sounds a complete sentence to me. It feels okay to leave the sentence like this as the information is complete. The focus of this sentence is the fact of making toys.