Put wrong fluid

Not the engine; you put it in the transmission. How much is "a little"? Had you done the opposite and put transmission fluid in the brake system, you would have a real serious and expensive mess. One drop of petroleum product, (engine oil, transmission fluid, or power steering fluid), in brake fluid will cause all rubber parts that come in contact with the fluid to swell. The ONLY acceptable repair for that is to replace everything that has rubber parts, and to flush and dry all the steel lines. If you have anti-lock brakes, that can run into thousands of dollars.

So we know glycol brake fluid and petroleum-based transmission fluid destroy the rubber parts in brake hydraulic systems, but the rubber seals in a transmission are made from a different kind of rubber. I think to be safe you should have the transmission flushed, which is a fairly common service now, but I don't think I would panic just yet. Don't wait any longer than necessary.