If the contract was made before 1978 -- and here's where it gets even more complicated -- you may terminate the contract any time during a period of five years beginning at the end of 56 years from when the copyright was originally secured or beginning on January 1, 1978, whichever is later (see why you may need a lawyer). So if your mother made the transfer in 1953, you would have five years, commencing in 2009, to terminate. The method of effecting the transfer -- it involves sending a notice -- is set forth in the Copyright Act at 17 USC 203(a)(4) and 304(c)(4) (again, the lawyer, the lawyer). It's possible you may also have a right to terminate the grant if the written agreement has been breached by the publisher -- for example, the publisher has failed to account for or otherwise honor the agreement. Ennyway... congratulations to your late Mom for getting a book deal based on a poem. We don't know much about poetry but we remember being twelve years old sitting around listening to a recording of this.
Additional Note: Dan from "A" Side Music, points out that the "subsidiary rights" you describe may not necessarily be a copyright assignment. Good point! As he notes, if it was simply an amendment to the book contract or a nonexclusive license to print the poem for a limited time in a publisher-owned magazine, you may not need to go through the "considerable hoops" required to terminate a copyright transfer. Check the paperwork and if it was a simple license, see if you can contact the publisher and arrange to terminate it (if it hasn't already terminated on its own).