The number of Representatives hasn't changed since it was raised to 435 in 1911 (1). However, the number of seats in the chamber isn't fixed.
Assigned seating in the house was abolished in 1913, after almost 60 years of assigning them by lottery. These days, as best as I can tell, seating is "first come, first serve" at all times. Democrats traditionally sit to the right of the chamber (as seen from the Speaker's podium) and Republicans to the left, but there are no actual rules requiring this.
As for people on the floor, I'd be surprised if the children were actually there during the vote (but I didn't see it myself). There's a list of people who are allowed to be on the floor, or the hallways leading there, but they're all government positions (Congress, the President, SCOTUS Justices, various heads of departments, governors, offical clerks, etc.) or staff thereof. When the House isn't actively meeting, then there's a slightly broader list, which includes the "persons employed in [the House's service]" (i.e. the janitorial staff), the press, and guests of the Representatives - the last is where children and friends would fall. (2) The key here is that all these people (including the kids) needs to be cleared out 15 minutes before the House convenes for business.
The gallery on the other hand, is a different story. There is a section of one gallery set aside for guests of the Representatives (3), and another set aside for families (and guests thereof). That's not on the floor itself, though, and seating up there wouldn't be considered part of the "446 benches" you asked about.
Finally, I couldn't find anything definitive about what happens when the Senators visit for the State of the Union or other event. I presume they simply set up more chairs, and I found one mention of them getting the first few rows of seats, but nothing definitive.
1 Except for the four years between when Alaska and Hawaii became states and when the next census reapportioned seats. There's also five Delegates and a Resident Commissioner who are also part of the House, but not actually Representatives.
2 The full set of rules that I summarized here can be found in the Rules of the U.S. House of Representives, 114th Congress, Rule IV, Parts 1-3.
3 This is why you need to get a gallery pass from your Representative or their office if you want to visit, and can't just go as part of a general tour.