There is actually a shuttle bus to the Brühl train station as well as the tram station "Brühl Mitte". It takes about 20 minutes and leaves every 30 minutes, tickets are 1,50 Euro for a one-way ticket.
A taxi to Cologne Central station will be about 50 - 70 Euros, depending on traffic and if it is pre-booked and so on. To Köln-Ehrenfeld may be around the same price, but a bit faster and easier to plan, since you must not drive into the city center.

Under normal conditions, the "bus - tram/train" connection should be safe.
But if you MUST catch ICE 12 at Köln-Ehrenfeld, a taxi would give you more safety imho.
It may be an idea to write to Phantasialand (if I remember correctly, you guys are staying there, right? Then use [email protected]) what they think your best option is. Maybe a local taxi company offers a "fixed price" or something.

If you do want to include an early dinner on that day (and return to Cologne with a rather large "minimum connection time", just to be safe), there is a nice beer garden about 600 meter from the Ehrenfeld station: https://herbrands.de/. They do open at 4 pm and you can reserve a table in the beer garden.