Translation:I am collecting the children from the school.
The question would be Cé mhéad páiste a bhailigh tú ón scoil? . “Have you collected” is an English past perfect, which is usually translated by an Irish simple past. An English “-ing” form would be needed to keep using the Irish verbal noun, e.g. “How many children have you been collecting from the school?” would be Cé mhéad páiste a raibh tú a bhailiú ón scoil? ; the ag changes to a (which causes lenition of the verbal noun) because the object now precedes the verbal noun.