At 20 months old, he will grow to be quite a bit taller with adequate feed but absolutely should not be ridden yet. He's not physically or mentally ready for it. You can read up on the physical issues here: http://www.equinestudies.org/ranger_..._2008_pdf1.pdf
Whether or not he'll be big enough for you depends a lot on how he's built. A stockier horse will be able to carry more weight comfortably.
The 3 year old may be able to do short bouts of light work, but I'd find someone smaller to work with her. She should really be closer to 4 before you start thinking of doing any serious riding with her, and she may not be a good match for a kid until she's even older than that. Young horses, even ones with good temperaments, can go through phases of testing their boundaries which can escalate quickly if not properly handled.
It doesn't sound like these horses are going to be ready for what you're looking for anytime soon, honestly.