If you buy a saver fare ticket (train-specific) well in advance, you might score a fare between €20 and €30 for the two of you, but any delay which prevents your boarding of the specific train you scheduled will void your ticket. To get a ticket valid on any train that same day, the cost will be double or triple that - and you can just wait until you are at the FRA airport station to buy that ticket if you prefer.
A reasonable gamble might be to build in a time "cushion" so that your "saver fare" train leaves FRA maybe 3 hours after arrival time. That should handle most flight delays. The day I checked shows a 14:52 train (1 change) priced at €30 and a 15:52 train (direct) priced at €30.
The 14:52 option with a change in Mannheim of 12 minutes involves moving between tracks 4 and 5, which share the same platform. But 12 minutes is very tight for ICE train connections. You may be delayed even longer if that first train is too late. I'd go with the direct train at 15:52.
Other choices:
- Talk to your airline about a rail + fly ticket, priced somewhere around €50.
- Just stay the first night near the airport, in Frankfurt or Mainz; it costs only a few Euros to reach these cities from the airport. Then wake up refreshed on Day 2 and catch your saver-fare train to Freiburg with no concerns about a flight delay at all. I may be missing some other option, but this might be a very reasonable choice in your situation. There's no sure way to know how you two will be feeling after a long-red-eye flight, and in Frankfurt or Mainz you'd have good options other than sitting and waiting. A short nap, a shower, the right plate of food, nice places to walk around... these sorts of things matter when you're dealing with jet-lag and fatigue.
