Complex sentence: But the ill Ferdinand kept his decision and when he died, 29th September 1833, Isabella was the legitimated queen... but she only was a child, and somebody had to assume the regency: his Mother, María Cristina de Nápoles.
Split into simpler sentences: But the ill Ferdinand kept his decision and when he died, 29th September 1833, Isabella became the legitimate queen. As she was only a child, someone had to assume the regency: her mother, María Cristina de Nápoles.