This sounds like the sort of itinerary that some guided tour would try to pull off: oh, look, now we're walking past the Chairman Mao Memorial Hall, and here's the Meridian Gate, and on your left you can see the... alright, now back on the bus, everyone!

It's definitely overly ambitious if you want to actually see anything and not just march past and take some snapshots. Especially with kids. Break it up, take things more slowly, and actually take the time to experience the places rather than just walk through.

As to the security checkpoints, at least as of last month, these were completely a non-issue. Unless you look very suspicious or have a lot of luggage, you can just keep walking. TSA-style security for an entire metro system that carries as many passengers as Beijing's does is simply impractical to implement. Security-related delays are certainly not a major concern, unless things have changed significantly in the run-up to the anniversary festivities.