The next time he breaks his thermos, I'm going to start sending him to school with a plastic drink bottle.
ーThe next time he breaks his thermos, I'm going to start sending him to school with a plastic drink bottle.
「次に水筒を壊したら、ペットボトルを学校に持って行かせます」
to break で「壊す」
thermos で「水筒・魔法瓶」
to send ... to school で「…を学校へ行かせる」
plastic drink bottle で「ペットボトル・プラスチックボトル」
ご参考まで!