The most important action to take in order to support the heritage language of new generations of Hispanics is to boost biliteracy instead of just pointing to bilingualism. Although often used interchangeably there are striking differences between someone who is bilingual and someone who is biliterate. A person who is bilingual can fluently speak in two languages, in this case Spanish and English. A person who is biliterate, can also read and write proficiently in those two languages.
In the case of young learners, while learning to read and write in two languages may look daunting, really there's no much difference in the process of learning to read in Spanish or in English. In both languages the child goes through three similar stages. The first, is called "emergent." At this stage, children are still unable to match speech to print but they begin learning to hold a book, turn pages, and sometimes they might even pretend to track words. At the next stage they learn the letters of the alphabet as well as the sounds that match the symbols. This is important as it is cementing what will be the child's development as a reader. During this phase it is important to offer them support by reading to them and with them providing plenty of opportunities for practice. The third stage is the instructional. The learner progresses from reading out loud to begin reading silently and also successively independent. Again support and encouragement are pivotals to promote efficient readers.
An important consideration in raising a biliterate child is to think about the linguistic characteristics of the languages. English is considered an "opaque" language, that is, many of its words follow ambiguous, irregular, and complex grapheme-phoneme conversion rules; which is one of the reasons for struggling-readers. Spanish is a "transparent" language, opposite to English its grapheme-phoneme conversion rules are simple and regular for most of its words. Considering these factors raising a Spanish-English biliterate child will be more effective by teaching the child to read in Spanish. This not only will support carrying on the heritage language, but it will also be more productive in the development of cross-linguistic transfer as they bridge from one language to the other.