Have not done Shanghai, but have done Beijing with a 1.5 year old and a 4 year old. We flew though, have no idea how long the train would take. It was very manageable, once you pace yourself and make sure that you have a buggy with you. We also hired a driver on days when we were trying to cover a bit more ground for 2 reasons; (i) so that the children's car seats were safely fitted (many Beijing taxis do not have seat belts) and (ii) we do not speak Mandarin (the doorman at the hotel would outline our itinerary to the driver, he also gave us his mobile in case we had any problems or changes in plans).
We went in late October, just after Golden Week. It was great, beautiful clear blue skies, neither hot nor cold and relatively clean air. Guessing thta Easter will be similar, but the air quality will probably be worse. We did find that things like Hutong bike tours etc. were out of the question, and we ate in hotel restaurants which is a bit boring, but the best thing for the children. Our 18 month old slept in the buggy while we were walking (one day we returned to the hotel for his nap). You can get an inexpensive guide as you walk into the Forbidden City etc., so that you can control the pace, get directions to the nearest bathroom etc. We also had a bag packed with snacks, juice etc.
It's not the same as going as a couple, but let's face it - holidays are different now. I think you should go, just choose the time of year carefully and give the plane a thought, there are some great packages available.
