Pop up Message when child is added and field on parent record changes

Mark's suggestion is what came to mind for me too. Your parent table would summarize (count) the children. But then you'd pass that count back to the child table using a lookup field.

Then once the child has info about it's siblings, you could give the User a heads up when that page is loaded, such as when the count is 9 or more via a Form Rule. Oftentimes, I'll wrap this into a helper formula checkbox for Form Rule readability. Something like:

// Children::Should Show xyz Warning
If (
    [Parent - # of Children] > 8,
    true,

    // Default to unchecked
    false
)

Then leverage that in your Form Rules to display a section with a "Text" Form Element where the HTML (Limited) box is checked. Or a maybe a Formula Rich Text field.

I usually lean on Bootstrap's Alert component styling for this type of messaging.

<p style="
    background-color: #cfe2ff;
    border-color: #b6d4fe;
    border-radius: .25rem;
    border: 1px solid transparent;
    color: #084298;
    font-size: 0.875rem;
    line-height: 1.4;
    margin-bottom: 1rem;
    padding: 1rem 1rem;">
    This is an <b>informational</b> message!
</p>

Hope that gives you some ideas!

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Brian Seymour
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