A child's passport often has a shorter validity period than an adult's - in the UK, for example, children's passports are valid for 5 years rather than the adult 10. Because kids aren't the ones planning trips or checking their own documents, this shorter cycle is easy for parents to lose track of entirely.
Why this catches parents off guard
Adults check their own passport before booking a trip out of habit. Nobody has that habit for a child's document - it sits in a drawer until a holiday is booked, at which point there's no time left to discover it expired eight months ago.
One extra wrinkle: growing faces
Some countries also apply stricter photo-matching scrutiny to children's passports as their appearance changes quickly. If a photo genuinely no longer resembles the child, it can cause delays at immigration even before the printed expiry date - another reason to renew proactively rather than right at the wire.
Track it under a parent's ownership
Add each child's passport as its own item, owned and reminded to the responsible parent - not the child. Set the reminder lead time to match the passport guide's 9-month rule, since the same 6-month-validity travel rule usually applies to children's passports too.