As others have said, your kids can come with you! If this is your first RunDisney event, I'm assuming you've never been to an expo before. When you arrive you'll need to go to one building to pick up your bib. Then you'll head to another building to pick up your race shirt. This is done on the main expo floor, so you can walk around and browse all of the booths from outside companies (lots to look at and lots to purchase!). After all of that, if you want any of the race-exclusive merch, you'll have to head to another building (if you go earlier in the day, this will require a spot in the VQ).
From my experience, the expo is much more crowded in the morning when it opens than in the afternoon just before closing. In the morning there are lots of lines (lines to pick up bibs, lines for characters, waiting for the VQ for merch, then lines in the building once your VQ is called), which obviously takes more time. The times I've gone in the evening, I've been able to walk right in to pick up my bib with minimal waiting. If you want race-exclusive merch, I'd recommend joining the VQ since that merch tends to go quickly on day 1.
If you just want to run in & quickly pick up your bib, that can be achieved in less than an hour (if you go in the evening). If you truly want to take your time and browse, I'd say up to 3 hours.
