From here. Mizuto Irido's dad Mineaki Irido married Yuni Ayai who is the mom of Yume Ayai. Yuni's last name changed from Ayai to Irido. For some reason Yume Ayai became Yume Irido too.
Is it common for children of the matriarch (well assuming heterosexual marriages) to change their last names too?
Generally, yes, children's last names will be changed when the parent gets married again. Legally it doesn't seem to be obligatory (children can keep the old surname).
Suppose a female A(Sato) married B(Tanaka) and had a child, left B with the child, and marries C(Suzuki). The default for the child's surname would change from Tanaka to Sato to Suzuki.
Source: This asks if anyone gets married again with their child's name unchanged, which suggests the opposite is considered normal to an extent. I don't see any statistics how often people change last names because of parents' remarriage.
