Yes. Newer (3.454 and later) versions of the Cactvs Cheminformatics toolkit (academic downloads at www.xemistry.com/academic) support the decoding of 1- and 3-letter amino acid codes into atomic connection tables (with stereochemistry).
Example command line, Python version (you'd probably script it instead of typing it), which writes two sample mini peptides as SD files:
cspy pycactvs>e=Ens('aa:AlaGluAsn') pycactvs>Molfile.Write('minipeptide1.sdf',e) [ens0] pycactvs>e=Ens('aa:AEN') pycactvs>Molfile.Write('minipeptide2.sdf',e) [ens1] pycactvs>
Do you really want just an atomic sequence? Or are you asking for the molecular structure in 2D or 3D?
In either case, you should keep in mind that back-translation (amino acid sequence to nucleotide sequence) will be ambiguous given that multiple codons code for each amino acid. You can use a tool like Backtranseq to convert from amino acid sequence to nucleotide sequence and it will use the most common codons for a given organism: http://www.ebi.ac.uk/Tools/st/emboss_backtranseq/
If you do want the molecular structure, you can then give the nucleotide sequence to a free, open-source tool like 3DNA: http://x3dna.org/
There is also a web-based version if you just want to test it out: http://w3dna.rutgers.edu/rebuild. Just select a DNA type and enter the sequence.
Both the offline and online versions will generate a PDB that can be looked at either in a text editor, or visualized with software like VMD or PyMOL. The w3dna online version also offers an in-browser viewer called Jmol for visualization.