Hamas used children to help them dig numerous tunnels into Israel and Egypt, a 2012 paper written for the Journal of Palestine Studies reported.
The paper, titled Gaza's Tunnel Phenomenon: The Unintended Dynamics of Israel's Siege says that little had been done to stop the phenomenon of child labor during the digging of the tunnels by Hamas in Gaza. In December 2011, the paper's author Nicolas Pelham accompanied a police patrol in Gaza and reported that "nothing was done to impede the use of children in the tunnels, where, much as in Victorian coal mines, they are prized for their nimble bodies." He continued and said that "at least 160 children have been killed in the tunnels, according to Hamas officials."
Here is the relevant excerpt:
A similarly cavalier approach to child labor and tunnel fatalities damaged the movement’s standing with human-rights groups, despite government assurances dating back to 2008 that it was considering curbs. During a police patrol that the author was permitted to accompany in December 2011, nothing was done to impede the use of children in the tunnels, where, much as in Victorian coal mines, they are prized for their nimble bodies. At least 160 children have been killed in the tunnels, according to Hamas officials. Safety controls on imports appear similarly lax, although the TAC insists that a sixteen-man contingent carries out sporadic spot-checks.
The abstract of the paper described the scene that Pelham found:
VISITORS APPROACHING RAFAH can be forgiven for thinking they have stepped back in time to the 1948 Nakba. On the southern reaches of the town, the horizon is interrupted by hundreds of white tents flapping in the wind. Instead of dispossessed refugees, the tents shelter the mouths of hundreds of tunnels, which for the past five years have played a critical role in providing a lifeline for Gazans hit by a punishing siege. Beneath the awnings, thousands of workers shovel heavy materials for Gaza’s reconstruction. Front-end loaders plow through the sands, loading juggernauts with gravel and enveloping the entire zone in dust clouds. Tanker trucks fill with gasoline from underground reservoirs; customs officials weigh trucks and issue the tax vouchers required to exit. The ground that Israel leveled in 2004 to create a barren corridor separating Gaza from Egypt is today abuzz with activity on and under the surface, as Gazans operate a tunnel complex that has become the driver of Gaza’s economy and the mainstay of its governing Palestinian Islamist movement, Hamas.
Hamas has invested millions in constructing tunnels to connect Egypt to Gaza in order to thwart border controls and thus importing fuel, concrete, weapons, and other materials needed to build tunnels and attack Israel. Hamas terrorists have emerged from a network of tunnels leading to Israel since the beginning of Operation Protective Edge and killed Israeli troops. The tunnels are also used as covert rocket-launching sites. Since the beginning of Operation Protective Edge 21 days ago, IDF forces have uncovered 31 tunnels leading into Israel. More than 1,000 Palestinians, mainly women and children, have been killed during the offensive. Hamas has, in many cases, refused to allow civilians to evacuate from areas eventually targetted by Israeli raids.