jhart4545, welcome to FT and the BA forum.

I think from your description your routing is IAD-LHR-FCO, then VCE-LHR-IAD?

BA does not through check on separate tickets, so you would have to retrieve and re-check your luggage. You need to pass through firstly immigration and then customs to get landside to re-check. How long this takes is, I'm afraid, a piece of string question - there is really no telling how long the queues might be for a (presumably) non-EU citizen. If you're travelling in Club World from IAD (or hold oneworld status) you'll qualify for Fast Track which should help (in theory). If you don't qualify, you could be looking at anything up to a couple of hours, or maybe as little as a few minutes. Customs is a walk through affair - you likely won't even see an officer. You could collect the bag by yourself and your family could go through the transit route if you preferred. This might be quicker than trying to get everyone admitted through the UK Border.

Everyone has to pass through security at LHR (or any other UK airport) when transiting/transferring from a flight from another country. The UK does not trust any other country's security. Even if you do an airside transit this will apply, so there's no gain either way. Again, the time needed depends on status and class of travel, but you would be extremely unlucky to encounter IAD-length queues at T5.

BA will try to sit you together on the same PNR, but this does not imply immediately next to each other - and with no young children it makes it much less likely. The new seating management system (FLY) should help with this somewhat, but if you really want to guarantee next door seats you need to pay.