And the next one who wants to know if it works :)

Daycare/preschool fees should be known in your area. Just check online. For us, the child benefit doesn’t even cover the daycare fee.

Then an amount x is saved, there are birthdays, you go somewhere (zoo etc.), swimming courses, soccer, gymnastics, clothing, food,

We now spend on average €450/month on food and drinks with 2 adults + 2 children (2+5).

And we also shop at Aldi & Co.

Given your financial situation, I would not overestimate the costs for a child. They don't need much at the beginning. We paid about €5000 for purchases (and that was a lot! It can definitely be done cheaper). If the mother breastfeeds, then you initially only need diapers, for which the child benefit is sufficient.

What has a much bigger impact is the loss of income. It is best to calculate. There is 65% [Elterngeld], but only for one year. Most mothers would like to stay home longer if they can afford it. It is sad when one has to go back because the income is necessary. And even then, you usually go part-time first, which also reduces income.
Just keep enough liquidity so that after the child and life and care situation, you can decide whether / when / how much you work with the child, and are not forced by your bank account!

Your answers are really helpful, thank you very much for that.

So assuming we have the first child, then I would have a net income of about €2080 (as of now), plus the parental allowance of €560 (?) for one year + child benefit which I am not including now as it compensates for the purchase of diapers.

That makes €2,640 net in the first year (the plan is that my wife, as soon as we have a child, will earn a little extra on weekends in the evenings on a €450 basis, but I am neglecting that for now, as it is not yet certain if it will really come to that).

If the second child comes a year after the first, the parental allowance continues for another year, right?

Assuming we would pay off a loan of €1,100 per month, we would still have €1,540 left for additional costs and living expenses.

Am I making any mistake in my thinking? That would of course only apply for the first two years; if the parental allowance ends, it would accordingly be less, which would then have to be compensated again (possibly through part-time work or something similar).

But basically I think I can live on €1,540 per month (with us).

Please tell me if I am too naive.

"If after one year comes the next one"... Yes, that's too naive.
1. You don't know how you'll cope with the first one. I had something similar planned but had a terrible pregnancy and have been fully occupied for seven months with a wonderful but demanding baby, so at no point could/wanted to imagine being pregnant again.
2. You never know how quickly it will happen. Especially since ovulation usually does not occur due to breastfeeding.

Better calculate with half of the parental allowance (32.5%) for two years. Then you'll be on the safe side!

Any side jobs I would mentally cross off for the time being. If it's a crying child, there will be no side jobs.

I will take your tips into consideration. It is really a difficult topic because you never know how things will actually turn out. I also don’t know how we will cope mentally with the burden of being "in debt" like this, since neither of us has ever financed anything before or anything like that. But I think, (without wanting to sugarcoat it like "it will somehow work out"), you have to take some risks for your dreams, and for that, our conditions – I think – are not the worst. And probably everything will turn out differently than you expect.