Greenpeace published a 15-page pamphlet expressing concern over a decision by LEGO to put Shell images on toys available at service stations operated by the energy company.
The advocacy group said Shell is building brand loyalty with the next generation of consumers, business leaders and politicians.
"This co-promotion is extremely worrying," Greenpeace said in a statement. "Shell has launched an insidious invasion of children's playrooms in order to prop up its public image."
Greenpeace said Shell and LEGO have a 40-year working history, with LEGO playing an "absolutely central role" in the energy company's social-imaging campaign.
The advocacy group praised LEGO for its environmental stewardship, noting it aims to rely on renewable energy for all of its needs by 2020. Greenpeace said, however, it's been pressing LEGO on its relationship with Shell, but has been so far ignored.
There was no statement issued in response by Shell or LEGO.