The court decision most relevant to the Woonsocket’s war memorial that displays a cross isn’t the Cranston West prayer banner decision, it is the 2010 Salazar vs. Buono Supreme Court decision. The original issue in Salazar was whether the Federal Government could transfer to a private owner a parcel of land within the Mojave National Preserve in California, on which a cross had been placed in 1934 “to honor American soldiers who fell in World War I”, in order to avoid any church-state entanglement issues.
In the plurality opinion that held that such a transfer was acceptable, Justice Anthony Kennedy also directly addressed the question of whether a cross could legally be displayed on public land as a war memorial…

The cross had stood on Sunrise Rock for nearly seven decades before the statute was enacted. By then, the cross and the cause it commemorated had become entwined in the public consciousness….
….a Latin cross is not merely a reaffirmation of Christian beliefs. It is a symbol often used to honor and respect those whose heroic acts, noble contributions, and patient striving help secure an honored place in history for this Nation and its people. Here, one Latin cross in the desert evokes far more than religion. It evokes thousands of small crosses in foreign fields marking the graves of Americans who fell in battles, battles whose tragedies are compounded if the fallen are forgotten.

This decision from just two years ago makes clear that bona fide war memorials that incorporate crosses on public lands are Constitutional, and there’s no question that the cross in Woonsocket, “erected in 1921 and…dedicated to four men who lost their lives defending the United States”, is part of a bona fide war memorial. (The succinct description of the memorial is from Tony Gugliotta of WJAR-TV).
A second court opinion worth reading on this matter is Judge Paul J. Kelly’s 2010 circuit-court dissent in American Atheists v. Duncan, aka the Utah roadside memorials case. In his dissent, Judge Kelly described how the “reasonable observers” constructed by the courts in church-state cases have a history of being anything but…

The court’s decision continues a troubling development in our Establishment Clause cases — the use of a “reasonable observer” who is increasingly hostile to religious symbols in the public sphere and who parses relevant context and history to find governmental endorsement of religion. Despite assurance from the Supreme Court that the Establishment Clause does not require us to “purge from the public sphere all that in any way partakes in the religious,” , the court’s “reasonable observer” seems intent on doing just that.

If the courts choose to ignore an opinion just two years old in order to declare the Woonsocket cross unconstitutional, then their rationale will be built upon an assumption that “reasonable” means growing ever-more cranky and irritated towards religious displays as time passes and that has been made to continually expand the scope of what is disallowed.