TAVISTOCK schoolchildren are all set to sing their hearts out this coming Sunday evening at the parish church, for their annual Songs of Praise.   Children from Tavistock Community Primary, St Peter's, St Rumon's, Kelly College Preparatory, Mount House and Tavistock College ha ve lined up their favouritesongs and hymns for the special service at St Eustachius Church at 6 pm.   Just like the televised Songs of Praise, the schools will be saying why they have chosen each song.   They will be backed by the parish church choir, and an anticipated service highlight will be a rendition of 'Wotcha looking for' byTavistock College gospel choir.   The priest-in-charge of Tavistock, the Reverend Michael Brierley, said: 'It's a rare occasion for the schools all to come together.  There's nothing like singing for bringing people together, and for bringing us out of ourselves.   'This service is a great expression of our unity, and children, parents, grandparents, friends, are all extremely welcome.'