Panel 1: We find ourselves watching the Pattersons celebrate April's first birthday as Mike tapes this for posterity. Since Elly has her eyes closed like an idiot, she doesn't see that April is reaching for the cake because she doesn't understand things like 'manners' or 'waiting' because she's a one year old child. As any major dude could tell you, this means nothing to Elly.
Panel 2: This is why she doesn't see the minor disaster coming when she asks her oblivious child if she wants to blow out the candle. April might be starting to know what individual words mean but complex sentences are beyond her to understand or formulate.
Panel 3: This is why she just dives on into the cake. She literally doesn't know any better. Mike is not much better because he's contractually obligated to ask what's going on despite it being obvious in order to set up a weak joke at Elly's expense.
Panel 4: Liz delivers the anemic punchline about April getting her wish. If she could wish for things, she should be wishing for a mother who understood children better than Elly does.
Summary: This "children do things on purpose only when I get messed up" crap informs all of the 'cute' kid humor Lynn concocts.