My ex husband is Irish and has English as his mother tongue. When I came to Sweden his two children were about 14 and 7. He had always refused to speak English to them, reasoning that he wanted them to speak one language well and not two languages badly. I have heard that was the prevailing school of thought at that time.
The children often mentioned that they were sorry they did not learn English as children because they feel they would have learned it better and without effort.
I became a stepgranny last week and the proud parents asked us if we would speak English to their son. I think that is a good way to do it. I learned two languages as a child and that went well. When I first came to Sweden and had to fetch my stepdaughter from school we had no common language so I would speak to her in Afrikaans (my second language) and she would answer in Swedish. The two languages are enough alike that it worked. Then my stepdaughter put post it notes on everything with names of things. So she actually taught me my first Swedish.