Keep one on one's toes

From Fresh Dictionary

English

Verb

keep one on one's toes

1. ((idiomatic{{#if:|, {{{2}}}{{#if:|, {{{3}}}{{#if:|, {{{4}}}{{#if:|, {{{5}}}{{#if:|, {{{6}}}{{#if:|, {{{7}}}{{#if:|, {{{8}}}{{#if:|, {{{9}}}}}}}}}}}}}}}}}}})) To keep one attentive, active, busy or alert.

   Four-year-old twins will certainly keep you on your toes!