DD2 just had her first birthday and DD1 is nearly 3, I often pop them in the bath and get back to the stove to tend to dinner while keeping both ears tuned for the bathroom. They both splash and sing really loud and carry on and DD1 always yells out if DD2 stands up or does something she knows is dangerous, I go running if I hear her yell, 'HAY-LEY!' in that stern tone of voice lol.
They're never left unattended for more than a few seconds at a time, I go and stick my head in to check on them or yell at DH to get off his bum and do some parenting while I cook.