You're right, the child support is based on your income. You pay on the $25K under the guidelines, as you've observed.
In Canada, an inheritance is a windfall, not taxable income, and not income for the purposes of child support.
If you invest the money and your income goes up because of it then with that higher income you'd pay more support. So indirectly an inheritance can bump up the child support. However, we're not talking millions of dollars in an inheritance. Even if you invested $100,000 at 4% return annual (which would be a pretty solid return in this market), your income would then go up by $4000 per year, add that to your other income, and pay support based on that total. But if you used the inheritance to pay off debts then your income wouldn't go up at all.
I asked about the size of the numbers because that's important. Let's say that I inherit a billion dollars and plop it into a chequing account that pays zero interest and I quit my job planning to simply spend from that billion for the rest of my life. Yes, my income is zero. But no judge would say that I should pay no child support.
Does that make sense? I'm here if there's more to discuss or if you have question or comment. If I've answered you I'd appreciate a positive service rating please. Ratings are how I get paid by the site for helping its customers.