Oh wow... the number of times I have sat at my computer literally screaming profanities at my sims as they wander around doing dumb stuff instead of following the very clear instructions I gave them to care for their toddler.
One trick I have found that works sometimes is play as the toddler. Another is don't be afraid to jump between sims to complete an action. Click into the toddler and ask mommy for food, click in mommy and cancel anything else in her queue until all she has up is Feed The Kid, click into the kid to make sure they don't wander off. Teamwork!