Children can learn breathing techniques as early as age 3-4, but the approach must be play-based, not clinical. Kids don't respond to 'activate your parasympathetic nervous system' — they respond to 'pretend you're blowing out birthday candles really slowly' or 'smell the flowers, blow out the candles.' The physiological effects are identical; the packaging makes the difference.
Age 3-5: Bubble breathing — inhale through the nose, exhale through pursed lips as if blowing a bubble (the slower and steadier, the bigger the bubble). Bunny breathing — three quick sniffs through the nose, one long exhale through the mouth (this is essentially a playful physiological sigh). Star breathing — trace a five-pointed star on the hand, breathing in on each upstroke and out on each downstroke. Age 6-10: Square breathing — trace a square on the desk or in the air (inhale up, hold across, exhale down, hold across). This is box breathing made visual and kinesthetic.
For tantrums and emotional meltdowns: don't instruct during the meltdown (the prefrontal cortex is offline). Instead, sit near the child and breathe audibly in a slow rhythm. Children's mirror neurons will begin to entrain to your breathing pattern. Once the peak has passed, gently suggest 'breathe with me' and do 5 slow breaths together. Practice the techniques during calm moments so they're available during storms. Schools that teach breathing exercises report 25-40% reductions in behavioral incidents.