Children learn best by observation and repetition, so give them good role models of worshippers and they will become good worshippers themselves. We live in a visual age where people, and especially children, are far more used to watching than they are to listening. It frustrates me greatly to think that an average 9 year old will sit for 3 or 4 hours at a time on a Saturday morning glued to children's TV, but the next day in our churches we often fail to hold their attention for more than a few minutes. Think how visual the worship in the temple of the Old Testament would have been, and how visual and evocative, for example, the act of communion is. Use contributions from as many different ages as you can, eg.
Teenagers especially prefer to be active contributors rather than passive participants.
Be practical about the length of time that children can concentrate for. It's better to have a shorter time of quality worship than a longer time that might deteriorate as some children switch off. Ask for feedback from different ages as to what they do and do not enjoy in worship. Ask too for feedback from other leaders that you can trust as to how they feel that it's gone. Persevere with all-age worship, and explain and teach as you go along why the church feels it is important to have times of worship with everyone present.