A short walk on hot cobblestones followed by a long security queue may be harder than a longer level walk. Ask for the longest day and hardest single section, not merely the total trip distance. Ask where children can sit, use the bathroom and take breaks.
St. Peter's dome involves 551 steps without the lift or 320 after it; Sagrada Família's towers exclude children under six. These should be treated as optional challenges, not requirements for a successful family pilgrimage.
Try a comparable family walk at home before departure. For medical, sensory or mobility needs, discuss the specific itinerary with the child's clinician and the operator.