The article The importance of weak ABO subgroups has a section on this, in addition to general information on what weak subgroups are. All quotes in this answer areas from that study.
Weak blood types
Weak blood types can be caused by a person having a mutation that leads to not expressing the type A or B antibodies as much, or by mutations in the genes coding for these that mean the antibodies they do produce react less when brought into contact with type A and B antigens for testing.
These weak phenotypes, in majority of the
cases result from the expression of an alternate weak allele present at the ABO loci.
Weak type A and B are different, and there are also different weak subtypes for each. For example, here is a case study of a patient with weak type A blood (much of the research is on that, as it is more common than weak type B): A weak blood group A phenotype caused by a translation-initiator mutation in the ABO gene.
Basically, weak type A or B blood donors's blood may be mistyped as being type O. If it is given to recipients with blood type O, these patients may experience blood agglutination which can be dangerous.
Identification of these subgroups is important because these donors may be mistyped as group O individuals. Wrongly grouped as O, weak subgroups of A or B red cells (if transfused to O group individuals) can show decreased survival.
This does not concern your case, as it is about weak type A, but I'm including this for completeness sake: One of the weak type A groups, Ax is special, in that people with this blood type should not donate for people with type A blood, as they actually have antibodies against the dominant type A antigen.
Similarly since Ax individuals almost always have anti-A1 antibodies in their serum. If clinically significant, they can lead to fatal transfusion reactions on transfusing their whole blood or plasma to group A individuals.
None of the sources I could find went into much detail what this meant for receiving blood transfusions. None mentioned not being able to receive type A or B blood if the patient is a weak type A or B.
Being mistyped as O when receiving blood is not going to lead to any major problems - however, it means the person will only receive type O blood and not type A or B blood. In situations with a donor blood shortage this might be a disadvantage.
The researchers of the study told the donors, so they would know that their blood could not be given to a type O patient.
All the donors were personally informed about their group and were given a special blood group card clarifying their donor as well as recipient status.
You might want to give your child something similar to carry around with them. Maybe the doctor/lab that identified the weak blood type has something suiting.