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BACKGROUND After tracheal tube insertion via various types of supraglottic airway devices, the distance from the tube cuff to the vocal cords has not been evaluated in children. OBJECTIVES The aim of this study was to evaluate the position of a tracheal tube cuff relative to the glottis in children when one of three supraglottic airway devices (I-gel, AuraGain and air-Q laryngeal airway) are used as intubation conduits. DESIGN A randomised controlled trial. SETTING Tertiary children's hospital. PATIENTS Children aged less than 7 years. INTERVENTION In vivo fibre-optic assessment and in vitro measurement. MAIN OUTCOME MEASURES The main outcome was the safety margin: the distance between the ventilation outlet of the supraglottic airway device and the beginning of the proximal cuff minus that from the ventilation outlet to the glottis. The maximum inner diameter of the cuffed tracheal tube that could be inserted, the fibre-optic view score and the oropharyngeal leak pressure were also evaluated. RESULTS The three devices exhibited significant differences in the distance from the ventilation outlet to the glottis (mean ± SD): I-gel 3.6 ± 0.6 cm, AuraGain 3.8 ± 0.7 cm, air-Q 2.8 ± 1.0 cm (P < 0.001). The safety margin was greatest with the air-Q and narrowest with the I-gel: I gel 1.9 ± 1.1 cm, AuraGain 4.4 ± 0.7 cm and air-Q 7.9 ± 1.1 cm. Using the AuraGain and air-Q, the cuffs of the tracheal tubes were predicted to be located below the glottis with one-size and two-size smaller tracheal tubes in all patients. However, using I-gel, the cuffs would be below the glottis in 69% (95% CI 49.6 to 84.5) and 29% (95% CI 14.0 to 48.4) of the patients with a one-size and two-size smaller tube, respectively. CONCLUSION The AuraGain and air-Q are well tolerated intubating conduits. The possibility of vocal cord damage is higher when the I-gel is used. TRIAL REGISTRATION www.clinicaltrials.gov (number: NCT03156166).