I don't know if you've started your journey already, about your queries:
1. Cafe Imran, Gharo, is a good option but that's only about 25-30 min drive from Karachi (Sassi toll plaza).
2. There are better eating options at Badin which is about 2.5-3.0 hours drive from Karachi (Sassi toll plaza)
3. Mithi has some good restaurants too, but don't expect anything with the likes of Karachi.
4. There are too many and too frequent dhaabas along the way, all the way till Islamkot which are a good option for tea.
5. Islamkot bypass has some good restaurants
6. Container Hotel Islamkot is a very good option to stay for the night
7. Rooplo Kohli is also a good option at Nagarparkar, but you have to decide if you want to drive all the way up in one go, which is about 7 hours non-stop from Sassi toll plaza, add another 1.5-2.0 hours rest stops to it.
The road from Karachi till Engro Coal mines just 20-min ahead of Islamkot is a brilliant road to drive on, however, from onward till Nagarparkar it's a narrow road with not so good carpeting. At least that is what I observed when I went there last year in September.
Islamkot is about 30 minutes ahead of Mithi, and from there, Nagarparkar is about 1.5 hours drive.
Your biggest problem, travelling with kids, is the non-availability of rest room (toilets) with commode seat IF your kids are not squat toilet trained otherwise they will cope easily.
Gadi Bhit in Mithi is a good place to visit at Sunset
Engro Coal Mines near Islamkot is a must to show the kids
Karoonjhar mountains, with Bodhesar Mosque (about 10 min drive from Rooplo Kohli motel) and Jain Temple are good sites in Nagarparkar, as well as Qaasbo village near the border.
You can plan your trip in a way that you may add Umerkot into it either going up or coming down. This way there will be a different route both ways. For example Karachi-Thatta (or Hyderabad)-Mirpurkhas-Umerkot-Islamkot-Nagarparkar OR Karachi-Thatta-Sajawal-Badin-Mithi-Islamkot-Nagarparkar-Islamkot-Umerkot-Mirpurkhas-Thatta (or Hyderabad)-Karachi
It's a good 2nights-3 days trip