I grew up speaking Cantonese, so all my toddler language (I've wet myself, please wipe my bottom etc) is in Cantonese. However once I started playgroup etc, English took over. Parents still spoke to me in Cantonese, but I'd reply in a mixture of the 2.
Parents made me go to Chinese school at the weekends, but I didn't learn anything of use.
The only time I found my knowledge of English was wanting was when someone used the term 'antimacassar' at school. Everyone else had learnt the term from their grandparents/parents, but I'd never heard the term before.
My Chinese Cantonese/Mandarin needs a lot of work. Technical terms confuse me, so watching the news involves a lot of guesswork, but a sitcom is fine.
However with satellite and internet TV, it's so much easier to keep a second language active. As I only spoke to my parents, my vocab was very limited. However with TV/films etc, you really do expand your vocab.
My youngest cousin in Holland had to start going to school aged 6, rather than 7, as Dutch wasn't his first language. That year was designed to bring him up to speed. It worked, as a result he's the only member of my family who can roll his 'R's.