Children under the age of 5-6 are typically more susceptable to overheating. The bodies physiological mechanism for heat regulation is in the brain and young children have not physically developed this heat regulation ability to maturity.
Hypercarbia is also common for people sitting in hot tubs.
A person sitting quietly and relaxed tends to breathe more shallowly. When this happens the blood contains higher levels of carbon dioxide. If they stand up quickly they may become dizzy and or briefly lose consciousness. The same situation can occur while sitting on your couch at home. Combine the effects of hypercarbia with an elevated blood pressure and heart rate and you can appreciated the possible consequences.