A three-year-old and a seven-year-old are not playing the same game, even when their hands are in the same tub. The older one wants a structure that stands; the younger one wants to touch what the older one built. Managing that gap is mostly a matter of layout and a couple of household rules.

Give each child their own patch of floor

The single most effective change is physical separation of builds. Two mats, or two ends of a rug, mean the younger child can reach their own pieces without walking through a wall the older one spent twenty minutes on. Shared pieces, separate territory.

Split the pieces before the session, not during

Divide the logs into two piles at the start. It feels bureaucratic and it prevents most of the arguments, because nobody has to negotiate for the piece they need at the moment they need it. Sets in the larger piece ranges make this far easier, and counts per set are listed at https://lincoln-logs.net/.

Give the younger child a job, not a smaller version

Three-year-olds are good at laying logs flat, sorting by length and delivering pieces. Put them in charge of the road, the fence or the supply pile. That is real participation rather than a consolation prize, and it keeps them at the same build.

The small parts problem does not disappear

If the younger child is under three, or still puts things in their mouth, the age marking applies even when an older sibling is present. Figures and short logs are the pieces to watch. Supervised sessions with a full sweep afterwards are the workable compromise.

Rules worth setting once

• Nobody demolishes a build they did not make
• Builds stay up until the owner says otherwise, or until bedtime
• Pieces go back into the tub, not into another toy box
• Figures are counted back in at the end of every session
• Building happens on the mat, so clear-up is one job not five

Deliberate collaboration works better than enforced sharing

Ask for one thing that needs both of them: a bridge between the two builds, or a wall long enough that neither has enough logs alone. Cooperation with a purpose lasts; cooperation as a moral instruction does not.

When the older child needs to be left alone

Some builds are ambitious and a small hand will end them. It is reasonable to give the older child a table rather than the floor, and to say plainly that this session is theirs. Not every activity has to be shared to be fair.

Do not buy a second set to solve it

The instinct is to buy another set so they each have one. In practice the two sets end up mixed in the same tub within a week, and the real constraint was floor space rather than piece count. Solve the space first and see whether the problem persists.

The gap closes on its own

Within a year or two the younger child can align notches and the age difference stops mattering. What matters in the meantime is that neither of them associates the set with being interrupted or being excluded. Layout and rules do that far more cheaply than another purchase.