As a rule, children learn their native languages easily, including bilingual children. What's hard for adult learners simply isn't hard for children.
In this case, yes, the relationship between the two languages is 1-to-2, but that's true of so many things, it's just a normal part of bilingualism. Consider the English pronouns he-she-it and Spanish él-ella, which is a 3-to-2 relation and split up differently. Or how there are more forms of verbs in Spanish than in English, or specifically two aspectual distinctions in the past tense (one is punctual, and one is prolonged duration, but both are past tense, like the English past, though that's not split).
On the other hand, there has been so much research on this topic regarding second language acquisition there is probably some experimental or observational research on children too, as a baseline. I haven't specifically looked into it, but I'd assume you can find it out there.