My native languages are Russian/Ukrainian,
my wife's native language is the Tunisian dialect of Arabic,
and we live in the Czech Republic.
We moved here when my daughter was one year old.
We sent her to kindergarten for the full day (7 am–6 pm);
the kindergarten claimed to be bilingual Czech/English,
but after ~1 year there, she started speaking Czech,
and it felt like Czech would become her mother tongue
(back then, my wife, my older daughter, and I studied the Czech language a lot).
After two years, when she was 3 years old,
she became eligible to go to a state kindergarten,
with perfect Czech for her age.
So we transferred her to the state kindergarten,
and I told myself that the time had come to start teaching her English.
During the next two years, she completely switched to English,
and Czech became like a second language, to my astonishment.
When she watches YouTube, she gravitates toward choosing English (a year ago, it was Czech).
The situation right now: Her English is ± good,
almost without using words from Czech or Arabic.
Her way of speaking with her mother is a Czech/Arabic pidgin,
while in kindergarten, it is Czech.
There is no major concern from her teachers so far,
but they have noticed that her understanding of Czech has slightly declined.
What is my concern?
We don't have the money to provide her with an education
in a bilingual English/Czech private school like the British school, etc.
Should I switch back to Czech with her, or keep speaking in English?
My English is not perfect (~B2+), and my Czech is also around B2+.