Even though with Hotwire you can specify the number of people, you still have no real control over number/size of beds...it is up to the hotel to offer you what they have available. If number/size of beds is a concern for your family, then it makes more sense to use a service like TravelZoo to identify specials that a hotel is offering and then book directly with the hotel. If you're driving, another option is to go ahead and book the cheapest you can Hotwire or Priceline, but bring along an aerobed so that just in case the bed situation doesn't work out, you have an option. My partner & I have done that on occasion when Priceline puts us in an Extended Stay America (that has hard, relatively small beds...one of us sleeps in the bed and the other in the aerobed).
And I would agree that it makes great sense to stay at a hotel which is convenient to a suburban metro stop (as service is typically frequent, it's cheaper than staying downtown and you'll likely have free parking). Although the cost of metro fares has gone up in recent years, a 7-day short-trip pass is $32.25 (which works out to less than $5/day). If you take a trip during rush that costs more than $3.25, then you'll have to pay an additional fare, but just schedule your time so you're not going in/out of the suburbs during rush hour.