David seems to be an effective babysitter - now
I hired him overnight while my parents slept a few times (the toddler was in it's crib due to wake up when he arrived) and he did everything he was meant to during the night so they woke to a sleeping baby with all it's needs met. He still isn't the tidiest of Sims but he does the babysitting job now when he is left alone in the house with the toddler and my parents don't come home to cranky children.
Doing the same thing with the red headed girl - Candy - resulted in having to get up in the middle of the night to fire her for playing video games downstairs all night while the toddler cried for food and a nappy (diaper) change.
Think I am just going to move an unemployed friend (live in Nanny!) into the spare room instead now lol