The balance of my child's two languages seems to be shifting. How can I ensure one language doesn't disappear?

          It is often the case that the strengths of a person's two languages tend to vary across time. A child may find it easier to speak English in some circumstances, Spanish in others and this may vary as practice and experience change. Sometimes the shift will be large. A child may stop speaking one of their languages while still being able to understand that language. Some children move towards speaking the majority language or their dominant language more and more, their minority language or less dominant language less and less. This is a naturally worrying event for many parents. It is often impossible and usually unwise to compel a child to speak a language. Sometimes, bilingual parents try to achieve conformity without conviction. When children are younger, one possible solution is to extend the range of language experiences in their less preferred language, for example, staying with grandparents or cousins, visits to enjoyable cultural festivals, a renewal in the language materials and other language stimuli in the home for that weaker language (e.g. videos, pop records, the visits of cousins). If both parents read to, or listen to the child reading before bedtime, or if the language of family conversation at the meal table is manipulated to advantage, then subtly the language balance of the home my be readjusted. Nevertheless, such children acquiring two languages early, will usually be able to communicate in both languages, often as well as they would communicate in one language.

SOURCE : A PARENTS' AND TEACHERS' GUIDE TO BILINGUALISM