There are plenty of children who refuse to speak their mother tongue although they understand it.
People always say it will come automatically, and to a degree it will - but especially where the mother tongue is only the language of one parent, and the other has the local language as mother tongue, it is not automatic that the child will eventually start speaking the mother tongue "like a native" at all.
DD has a girl with an American mother and German father in her class at her German secondary school. I've never met the mother but met the father while we were dropping our DDs off and waiting for them to leave to go on a trip - he asked me how I made DD speak English, as his American wife has always spoken English to their DD and she has never spoken English back, but has always replied in German. Now at age 12 they do English at school and unlike my DD his is not getting effort-free As in English but Bs and Cs... She understands the language but won't "produce" it, which also means she can't spell it and when she writes the grammar is not as automatic as for a child who actively uses the language.
The answer to how I've made my kids speak English while otherwise totally immersed in German and local dialect (they have been in the local system since age 3 and we have no native English speakers locally) is that I've insisted on English when speaking to me the same way I insist on them saying please when they ask for things... When you are actually 100% immersed in another language outside the home I actually think that is the only way if you want both languages to be equally strong - just as I ignore my kids asking for things, or if necessary remind them to ask nicely, if they ask rudely, thus ensuring they ask politely, I treat speaking to me in German the same way...
When they were under about 4 all TV was also cbeebies, but as they have got older it is also nicer if they can discuss TV with friends so now I have no "rule" about TV. DH and I rarely watch German TV though (because it is mostly crap - DH is German and prefers English TV). As a family we watch films in the original language if that language is English or German - so it works out that we watch some in German but most in English.
I still read to my kids every night that I do bedtime (I work a few evenings) and I always read in English - I think this is massively important and it is the main reason I still read to the older 2, who are perfectly capable of reading to themselves. DD reads voraciously but mainly in German (though if a book she fancies reading is in English she will read it). DS1 isn't a great recreational reader but loves the Beano, which I have a subscription to specifically to keep their "kid English" topped up, and reads that cover to cover in English, which is all good IMO :o
When we had a CD player in the car up until a few months ago I always had a new English audio book for any anticipated longer journey, and everyone listened to those together - as they got older Roald Dahl and Famous Five were good, earlier nursery rhyme collections and Percy the Lighthouse keeper, Mog etc.
So read to them and treat replying in the language they are spoken to in the same way as you regard saying please and thank you, and watch English TV and have English audio books in the car, would be my tips :o I do think it requires a degree of work but absolutely does not require any formal "teaching".
Good luck.