The date of registration of the births was 5 July. There were additional permissions and fines required to register a birth after more than 3 months, so, as commonly happened with late registrations, it seems the birth date has been adjusted to avoid the fines.
The census question was who resided here on the night of 31 March, and I think there is every reason to trust that return. The form may have been filled out in advance, and then the birth occurred and they had to be added after the servants.
I'd say they were born on 29 or 30 March.
As they were Catholic you may well find a baptism recorded within a day or two of birth which would help confirm this.