This is where your thinking is fundamentally flawed and you are not even close to comparing apples to apples.A debt free property worth 700K + appreciation over next 15 years vs 100k invested today for them in 15 years ?
As @Sarenco has already pointed out to you, your rental is generating a return of ~€3k net. This means that your income is paying down the bulk of the mortgage, not the property itself. And with the level of debt you have in loans, credit card and ppr mortgage, it is loss making.
You currently have debt commitments of ~€5k per month. Unusually both your mortgages go well past your retirement age which is a major red flag. If you were to try clearing these by 65, your monthly commitment would be €5.7k.
Your major problem is cashflow and its only going to get worse in 4/5 years when your kids do start university. You don't have the income to sustain the debt and cost of university.
If you sell the rental (which you absolutely should), it frees up ~€5k per month which can be invested for the next 15 years in something that is more easily accessible e.g. shares/ETF's. It also allows you to easily fund university from your income.
I would also imagine that you won't have €100k left over to play with. There is surely a significant CGT bill on the rental and you should be clearing all debts, not just the ppr mortgage. But even that doesn't change the very obvious need for you to sell the rental