The couple of nine years, who have sons, Sasha, seven, and Samuel, five, have never put marriage at the top of their list of priorities, reported OK! magazine.
The boys are apparently keen to see their parents walk down the aisle and have started helping them tentatively plan their nuptials.
"Marriage was never a big deal to Naomi or Liev. But now their boys are asking why they aren't married, so they're gonna take the plunge. They would do anything for their kids, and they think it will be a fun family thing to plan together," a source said.
"It will be a small, intimate wedding, with just immediate family and friends. Their sons will be the only members of the wedding party," the source added.