This report is well known as it's a footnote on the Wikipedia article on this case. And this report has not held up well. Stickel is disputed by those who found that the none of the junk under the preschool dates AFTER 1966 when the concrete slab of the Preschool foundation was poured. And there was nothing to support or lines the tunnels which you would need to keep the tunnels from collapsing.

From Wikipedia

Continued allegations of secret tunnels In 1990, parents who believed their children had been abused at the preschool hired archeologist E. Gary Stickel to investigate the site. In May 1990, Stickel claimed he found evidence of tunnels, consistent with the children's accounts, under the McMartin Preschool using ground-penetrating radar.[39]

Others have disagreed with Stickel's conclusions. John Earl wrote in 1995 that the concrete slab floor was undisturbed except for a small patch where the sewer line was tapped into. Once the slab was removed, there was no sign of any materials to line or hold up any tunnels, and the concrete floor would have made it impossible for the defendants to fill in any tunnels once the abuse investigation began. The article concluded that disturbed soil under the slab was from the sewer line and construction fill buried under the slab before it was poured. Further, Earl noted that some fill from beneath the concrete slab was dated to the year 1940.[16]

W. Joseph Wyatt's 2002 report concluded that the "tunnels" under the preschool were more plausibly explained as a rubbish pit used by the owners of the site before the preschool's construction in 1966. Materials found during the excavation included bottles, tin cans, plywood, inner tubes, as well as the former owner's old mail box. Only three small items found near the edge of the concrete slab were dated after 1966, which Wyatt suggested were most likely dragged into the pit by rats or other scavengers. Moreover, Wyatt speculated that Stickel's conclusions were colored by his collaboration with the parents of the McMartin children.[40]