On the bike I use a Zefal HPX-4 frame pump. We all have fat tires and it fills them quickly. In the garage I use a Blackburn something or ither floor pump, with an integral gauge. For either, there are lots of good options out there.
A way to carry things is important, whether it's an under-the-saddle bag, a rack/trunk bag, a basket, or whatever else suits your fancy. I like having extra capacity in case we go into a shop or otherwise pick things up along the way.
Ways to carry water are important. Everyone should be able to carry at least one water bottle for themselves. Insulated bottles are nice, but I just use a simple stainless steel bottle.
I think a bell is a very good thing to have. All of our bikes have a Japanese Crane bell. My own bike also has a little jingle bell hanging from twine, for a little less "insistent" ding.
There's a million cool things you can attach to your bike. Kids may find different things important than adults. Have fun with it.