My 2 DC are bilingual French/English. They are 11 and 13. We live in France and they go to French school so their French is definitely on a par with their peers. (And we have proof in their results).
English is spoken with me at home, with family on numerous occasions (they spend 5-8 weeks in the UK every year) plus they read for pleasure in English and watch a lot of TV and YouTube in English.
Their English is also definitely on a par with native speakers their age - in reading, comprehension and speaking. They have no accent and use normal language when speaking, thanks to their huge exposure to the language. DS1s written language is brilliant (thanks to reading). DS2 doesn't write so much but surprises us all with his spelling ability.
At age 4:
DS1 was behind peers in both languages (later diagnosed with SN) but we worked hard on exposing him to as much variety as possible and now, age 13, you wouldn't guess he had struggled.
DS2 has always excelled at both languages (to our surprise after DS1!)
Basically, it took a hell of a lot of work from me, the parent speaking the minority language. I read to them every night. I made them watch TV and films in English. I talk to them all the time and I have had to make sure I don't "dumb" my language down but speak normally. I model correct language, i choose when I correct their mistakes (not too much to dishearten them but enough so they don't get entrenched saying the wrong expression). I also insist on them talking to me in English despite me being bilingual in French.
It's hard but absolutely worth it.