So you have a child bike seat that's so old and had so much use that one key part has fatigued and broken. Every other part on that bike seat has the same amount of age and use. What makes you think that none of the other parts are nearing the end of their lifespan?
I'm not real big on child bike seats at best. I'm thinking that if you love that child the bare minimum you should consider is replacing the whole baby seat.