Parents often expect the second child to be easier because the skills are already learned. What changes instead is the structure of the demand, and that is a different problem from inexperience.
The binding constraint becomes simultaneity
With one child, the scarce resource is time, and time can be rearranged. Tasks move later, sleep is borrowed, and most days can be reordered until they fit.
With two, the scarce resource is being in two places at once. A toddler needing the toilet and a newborn needing feeding are not sequential problems that patience can solve.
This is why households with two children often report that logistics, not fatigue, is the thing that broke. The workload rose modestly while the number of collisions rose steeply.
The first child loses exclusivity, not attention
An older child usually receives similar total attention after a sibling arrives, but almost none of it uninterrupted. That distinction matters more to a young child than the total does.
Regression in a first child, such as returning to baby speech or wanting to be carried, is generally a bid for the earlier kind of attention rather than the earlier quantity.
Short periods of undivided attention tend to settle this faster than longer periods of shared attention. The variable the child is responding to is interruption, not duration.
Parenting becomes less deliberate
First children are usually parented with more observation and more anxiety, because every stage is unfamiliar. Second children are parented with more experience and considerably less scrutiny.
Much of that reduced scrutiny is harmless, since the parent now knows which things resolve on their own. Some of it means a genuine issue is noticed later than it would have been.
The practical response is not more worrying but keeping the same checkpoints as before, particularly routine health and development reviews with a professional who sees the child fresh.
The sibling relationship begins asymmetric
The older child remembers a household without the younger one, and the younger never knows anything else. That asymmetry shapes the relationship for years and is not a fault in either child.
The younger child usually adapts to the older one's schedule, which advances some things and delays others. Second children often speak in longer utterances earlier and sleep less predictably.
Comparison between the two is almost unavoidable and rarely useful. The gap in circumstances is large enough that developmental differences between siblings carry very little information.
Household systems stop scaling
Arrangements built for one child tend to fail rather than stretch. A single bedtime routine becomes two overlapping ones, and a car that fitted one seat may not fit two.
Families usually rebuild these systems in the first months rather than adapting them, and the rebuilding is the bulk of the difficulty parents describe as the second-child adjustment.
Once rebuilt, the systems tend to be more robust than the originals, because they were designed under pressure for two children rather than improvised for one.