Dedicated children's content was taken off BBC1 and BBC2 in January and is now only broadcast on digital channels CBeebies, for children aged six and under, and CBBC, which is aimed at six to 12-year-olds.
But the Trust's report said that 2.1 million children, aged four to 12, still watch BBC1 and BBC2 every week but do not watch CBeebies or CBBC.
The Trust said: "We would like to see more concerted action taken by other BBC TV, radio and online services to consider children as an audience for mainstream output and to continue to seek opportunities to include relevant BBC Children's output as part of their offer."
Both CBeebies and CBBC shut down at 7pm, but the Trust suggested that, with 4.5 million four to 12-year-olds still watching TV after that time every week, some CBBC programmes that would appeal to a mainstream audience should be shown later on other channels. In recent years, some CBBC programmes such as Horrible Histories have been shown on BBC1.