Title text: I bet my future kids will read this someday. DEAR FUTURE KIDS: how did you get internet in the cellar?
A common theme of xkcd is that one never feels that one has "transitioned to adulthood", in the sense of actually attaining the seriousness and sense of responsibility that children imagine all adults to possess. Here, the author illustrates this by imagining Cueball and Megan taking on the ultimate "adult responsibility" — having a child, treating it as they would any other engineering project. Disassembling a project to check the parts is an activity that is appropriate for a self-built computer or robot, but disassembling a child will be impractical and possibly lethal for the child. Megan also shows her lack of child experience by holding the baby upside-down by the foot, which usually isn't a good idea (it also displays Megan treating the child as an object rather than a human being).
The title text implies the author will have kids someday. It will be surprising if they read this comic, not just because it will give them an unflattering look into their father's attitudes on having children, but because he plans to lock them in the cellar where there will be no internet access. This is a reference to Kaspar Hauser:  , who is a boy that was claimed to have grown up in a dark cell in Germany in the 19th century.
- It doesn't seem right that we're old enough to have kids.
- [Megan holds a baby upside-down by one leg.]
- Megan: Sweet! We made a baby!
- Cueball: Are we sure we did it right?
- Cueball: We should disassemble it, check all the parts, and put it back together.