As answered in Sage Advice (see How to decide what creatures are summoned?), the spell is expected to work like this: the caster chooses the number and CR of creatures, and the DM fills the request (presumably with something thematic for the location or situation).
I know it's dangerous to speculate as to intent, but I think this is probably meant intentionally as a balancing factor so that — pixies aside, even — game designers wouldn't later need to worry about introducing some monster with a cool ability which happens to be super-broken when accessible to players through this spell.
So, while you say "I want to be fair and not just never allow a druid the chance of summoning pixies", I don't think that's actually not being fair at all. It's exactly fair and is as intended. There are some things a DM could do within the reading of the rules that don't seem fair, particularly with the "or lower" option — as written, you could ask for "one creature of CR 2 or lower" and get a single boggle — I think that'd be kind of a jerk move*. But declining to select a fey with an ability which you as the DM thinks gets out of control? That's... completely fair.
I'm playing a mid-level druid in a game right now which works this way, and it's generally been great. Because we've had some dealings with the fae in the game, in one particularly fraught battle where we were defending something the fae held dear, my DM did have pixies answer the call, and we did have a crazy t-rex battle and it was awesome. But as a player, I don't expect that to happen all the time.
As for distance: yes, they can cast and leave the area. But, I note that you say this in the comments:
the pixies fly far away and the caster is now a dinosaur with War Caster.
... and have two things to note. First, from the spell,
They obey any verbal commands that you issue to them (no action required by you). If you don't issue any commands to them, they defend themselves from hostile creatures, but otherwise take no actions.
So, if you do bring some pixies to the mix, "polymorph me, fly away and hide" would be a perfectly fine command. But once the pixies are out of earshot, they're not taking any more actions.
Second, unless you have a specific "dinosaur who has War Caster as a feature" monster in your game, the caster can't be a dinosaur with that feat, because you don't get your feats while polymorphed.
* (and the DM should at least say: no CR 2 fey is around to answer the call; would you prefer more of a lower-CR creature?)
