Panel 1: Now that things have wrapped up, John solicits praise from Elly on Liz's behalf because she's getting to be a good little carpenter.
Panel 2: He practically bursts with pride about how she spent the afternoon in the workshop and enjoyed every minute of it.
Panel 3: As they walk away trailing sawdust, John horrifies Elly by saying that day by day, she's getting more like her dad.
Summary: The joke is pretty much on Liz and Elly, sadly. This is because John's fussy nature and fear that tiny hands will wreck his toys soon take care of whatever interest Liz might have in his interests that his putting up his much birdhouse up didn't kill. Today, it's sort of helping out. Tomorrow, she'll be in denial about standing there in the background doing nothing because she doesn't want to get yelled at.