How much should I pay for a Freemium?

Asking for money in a freemium is not scandalous, if it is not mixed with too many advertisements, and within limits, which often are not there yet.

When you play something, you should always think that it is the result of work, so merits some support. Then how much is that game worth to you?
- Is that something that brightens your day for 10 minutes every day? Then think about it like a daily cup of coffee.
- Was it a great experience for 2-3 hours? Then it was like watching a movie.
- Is it something nice for your kid...

Then think about the limits. Big games on PC and console costs around 60-70 €/$ when they are released. Some freemium offer repeated purchases, for a one time use (if you restart the game you can't get it back), at 99€/$. That is much too much. That means that in just 4 purchases you could basically buy a console and a game for the same price. And you still haven't purchased the game...
Now some games offer one or several purchases giving or not bonuses, or simply removing ads, between 1€/$ and 10€/$. If they are definitive, it's perfectly fine.

And a tip for all parents: never allow your purchases on any phone or tablet to be without a password. When asked if you want to save your password to make future purchases easier, always say no. BTW it is also a good thing without kids, it gives you extra time to think about your purchase.