As far as cost, you might find an opportunity to spread the cost of a charger to other devices, particularly true for more common place NiMH rechargeable cells than li-ion 18650s. If you need / can use AA or AAA rechargeable cells for other purposes, you have a chance to save some money over time.
Brightness is hard to define. You'll read a lot about throw, hotspots, spill and flood around here. For a general purpose light you don't generally want the world's best thrower nor an all flood light but something in the middle. The TK11/12/15 I would consider in that realm as is the P Rocket, and so would be AA powered lights like the Quark AA2, Fenix LD20, LD25 or the 4-cell LD40. There are tons of different lights in single or double AA size format that would make ideal first lights. Here are a few comparison shots between some picked as examples of what you get as output for size and battery power rather than as specific recommendations. Try opening up two windows side by side to compare a couple at a time:
AA
1xAA Fenix LD10 R4
2xAA Fenix LD20
4xAA Fenix LD40
2xCR123 or 1x18650
Fenix TK11 (simpler UI than TK12)
Fenix TK12
EagleTac T20C2 Mark II (no tunnel shot to compare but you can choose the long hallway shot for all of the above for comparison)
Sadly the site I'm pointing you do doesn't have every light under the sun, but at least you can get a feeling for different levels of output.
Size: As for the TK11/12/15 or EagleTac or MG P Rocket and the like - they will all be a little bigger. Thicker around and longer. You may not find carrying one on your person nearly comfortable day in day out, but if you have no plans to do that, they by all means buy big. I always try to talk people into buying a smaller first light because I'm sold on having a light on me at all times. I'm a software developer who moonlights in other areas - I find my single AA light fits right in with my front cargo pocket in shorts, or in my dress slacks or suit jacket inside pocket. I would never carry my 18650 lights in a suit jacket! Others may.
UI: On user interfaces, some like simple, some don't mind complex. Like beam shape and size, these are very personal things. If you just want on/off simplicity, buy a light like that. If you don't mind various output levels, perhaps a strobe mode or two, then buy a light with those features.
Ive got two young kids and no matter how careful I am at hiding it, they are bound to get their hands on it and drop it etc.
Safety: One thing about 18650 (and other sizes) lithium ion cells - they are not just another type of relatively innocuous battery.
Mistreated, they can get in a state where nasty things can happen including fire and explosions. You've got kids. Consider this, please. I've got 10 and 13 year old sons now and only fairly recently have I felt comfortable having li-ion rechargeable cells be in the house as I can trust them now. I want my kids to use flashlights and have access to them - so most of ours are AA or AAA NiMH powered.
If you do buy a li-ion powered light please be sure to visit the Battery subforum here, read and ask a few questions just so you know what you are getting into before you take that leap.
Here's a recent thread where someone did just that.