I've posted this elsewhere, but this was a helpful statement on the Facebook Covenanter forum about the regulative principle of worship applying to all worship, yet with a difference that answers some of the difficulties often raised in family and private worship if the RPW applies there. Posted by RPCNA theological student, Pacific Coast Presbytery, Seni Adeyemi:
Times family worship are far different from public worship, but it [RPW] still applies to both. From the moment of the call to worship, the congregation enters into the special presence of God, the heavenly Jerusalem, mount Sion, an innumerable company of angels, etc. That is why during that time period between the call to worship and the benediction, only the elements of worship are to be done (reading and hearing of Scripture, prayer, psalm-singing, preaching, sacraments, etc.) and nothing else. During family worship there is no special entering into the presence of God like in public worship. Thus there is simply a duty to do the specific acts of worship (pray, read scripture, sing psalms); but there is not a special time during which only those things may be done. Thus acts of non-worship (reading man-made devotionals, discussion, etc.) may be done as helps during those times, because there is nothing special about the time called ‘family worship.’ When you read Ryle you are not worshipping God, even though you do it during that time you call family worship. But you are not so bound to only do formal acts of worship during those times because there is no entering into the special presence of God. Properly speaking, the duty of family worship is only fulfilled insofar as the formal acts of worship are performed. So the RPW does govern both public and family worship, albeit in different ways. The RPW governs the acts of worship in family worship (i.e. only prayer, psalm-singing, and scripture-reading are counted for worship), but it governs both the acts and the time set apart for public worship, such that nothing other than formal acts of worship may be done in the special presence of God.