I was a primary TA / helper for over twenty years, during which time education, teaching and expectations changed greatly - and are still changing.
In the '50s I went to a good grammar school, but only obtained three 'O' levels, failing many others, including maths, French, Latin, chemistry and physics.
Today children know things that even the best scientists didn't know fifty years ago (plate tectonics, for example), so rather than hide your lack of knowledge in specific areas, explain to your children that the world has moved on at such a pace that you haven't kept up with everything, and invite the children to help to keep you up-to-date.
Books by David Crystal and Lynne Truss should help with English grammar and language (which is still evolving, anyway).
Watch TV programmes such as Sky at Night, "Click" (tech and computers on the BBC News Channel), Horizon, natural history, etc to try and keep up with things, and encourage children to take an interest in them too.