Because their routes differ. Children who need a consular TD visa incur application costs and travel to the appointment, while visa exempt children generally do not. Translations of foreign-issued certificates add cost for some children only. Confirm current fee amounts with the relevant agency.
Budget per child, not per family
A per-child estimate makes the difference visible: passport renewal for one, a consular application and travel for another, a translated birth certificate for a third. Families that budget a single lump sum tend to be surprised by the child whose paperwork is the most involved. Where documents must be requested from a foreign registry, allow for both the fee and the shipping.
Name the payer for each line before the estimate is agreed. Employers commonly cover costs tied to the principal's professional entry and stop there; dependent costs then fall to the household, and a family that assumed otherwise learns it after committing. Ask the employer in writing which dependent items it will reimburse.
Then take each child separately: passport renewal, a consular application where one is required, registry retrieval, and translation. Foreign registries charge their own fees on their own schedule and answer to nobody's deadline. Confirm current government fee amounts with the relevant agency at the time of payment rather than from an older quotation.