Immunotherapy for Solid Tumor Malignancies in Pediatrics Using Interleukin-15 and -21 Armored Glypican-3-specific Chimeric Antigen Receptor T Cells

This study is testing a new type of CAR T-cell therapy — a treatment where your own immune cells are engineered to fight cancer — targeting a protein called GPC3 in children and young adults with solid tumors. The engineered cells are also "armed" with proteins (interleukins 15 and 21) to help them work better. **You may be eligible if...** - You are a child or young adult with a solid tumor that expresses a protein called GPC3 - You have reasonable physical functioning (Lansky or Karnofsky score 60% or above) - Your expected life expectancy is more than 16 weeks - If you have liver cancer specifically, it must be at Barcelona stage A, B, or C, with adequate liver function **You may NOT be eligible if...** - You have a known allergy to mouse-derived proteins - You have had an organ transplant - You are HIV positive - You have an active serious infection Talk to your doctor to see if this trial is right for you.