Is it possible for children to learn English as well as their mother tongue?

It is entirely possible to learn well!

However, it is absolutely impossible to be as good as your mother tongue!

There is a simple reason. Mother tongue, with genetic phonemes and ontological inheritance, has been in contact since childhood, and daily communication and learning have accumulated over time. Coupled with the enlightenment of teaching, the accumulation and edification of traditional culture, as well as the influence of customs and film and television media, this kind of language and culture accumulation that goes deep into the details of life is far from being comparable to foreign language learning.