Accurate characterization of X-ray beams underpins radiation protection, dosimetry, and shielding design. This study presents a Monte Carlo simulation framework using the Particle and Heavy Ion Transport Code System (PHITS) 3.35 to characterize the photon energy spectra of a cabinet X-ray irradiator (Hitachi MBR1618RBE) at 80, 100, and 120 kVp, spanning the voltage range used for pediatric chest and abdominal radiography. A tungsten anode tube model (20° anode, 0.8 mm Be window, 0.5 mm Al + 0.1 mm Cu filter) was implemented, with spectral fluence scored via the TTrack tally at z = 7 cm (2 × 10⁷ histories/run). Simulated mean photon energies were 57.9, 62.0, and 67.6 keV. First half-value layers (HVL₁), derived from National Institute of Standards and Technology Photon Cross Sections data, were 3.08, 3.25, and 3.43 mm Al, with homogeneity coefficients of 0.970–0.989 confirming polychromatic beam quality. Depth-dose, off-axis, entrance surface air kerma, effective dose, and lead-shielding quantities were derived by analytical spectral convolution and published coefficients rather than full Monte Carlo transport, yielding bounding, illustrative estimates. Pediatric effective dose (5-year-old phantom, source-to-surface distance = 25 cm, National Radiological Protection Board W14 factors) was 0.35–5.19 mSv per examination; these are not directly comparable to clinical diagnostic reference levels. Required lead thicknesses at a 1.5 m wall under conservative occupancy assumptions were 8.3–15.3 mm Pb (public) and 5.6–11.3 mm Pb (occupational). A PHITS implementation note distinguishing vacuum (code 0) from outer void (code 1) is documented for practitioners. No independent experimental validation was performed. These results characterize the simulated source term and illustrate a PHITS-based workflow, not validated clinical reference values.