I would think that crazy gle would work, but that is a lot of letters to glue... I really can't think of an easy way. How much traffic will the area have? If it is not a lot then maybe you can glue in 3 or 4 spots around the letter, and that should hold it. If it high traffic or kids will be pulling at it, then you would probably need to glue most of the way around the letters - no easy way out.
Alternatively, double side sticky tape might work too... but a truly persitant child might be able to pull a letter out.
Good luck!