If money is a concern, the Bolt bus or similar is really great and about $50/person. I have taken it several times and there are always some cool older people on it who look like they are doing just what your ILs want to do

The drive up to NYC wouldn't be too bad - usually about 4 hours from our house in Arlington if traffic is normal and I drive 70 in NJ like everyone else - but the parking could be difficult (and is sure to be expensive) and the drive BACK will be awful because they will be exhausted. The Bolt bus leaves from Arlington and Bethesda, and the Peter Pan in DC is also perfectly fine (much better than the bad old days when I first started taking it - though not as nice as the Bolt). It's sad that trains are so bloody expensive - that would be the most comfortable option.