The Multiclass Feats and Paragon Path mutliclassing still offer plenty of ways to make an 'Arcane Trickster' type character work. Focus on Dex and Int when assigning stats, grab the Multiclassing feats as they become available(possibly asking your DM if you can get Stealth instead of Thievery for Thief of the Shadows). Throw down your extra 2 feats on Skill Training, Armor or Weapon Proficiencies, and you're pretty much already good to go.
At 1st level, you'll already have more than in 3.x because you'll already have Sneak attack and extra skill training. At second, you can your first proficiency, making you a viable threat with that dagger or letting you stand up to a few more attacks than Cloth would normally allow.
You progress slowly, but by 10th level, you can, for instance, move from cover to cover without taking a Stealth penalty for movement and toss out invisibility-type spells, knock an adjacent enemy unconscious with a fluid blow once per day in addition to your healthy selection of spells, and blind an enemy with sand once per encounter, all at the cost of a few Wizard abilities. You don't get as much sneak attack damage, but if all you want is damage, then sneaky is something you are not.
Now you have the option of multiclassing at 11th, or taking a Paragon path for either the Wizard or Rogue classes. The Paragon Paths add plenty of options, but let's take a look at what you'd look like if you continued multiclassing.
At 11th, you can take an at-will, such as Riposte Strike, which allows you to fight in melee if the enemies close in, giving you a counter attack if they continue to press you. Over the first two levels, you can also make your Rogue Encounter and Utility powers a complement to, rather than at the expense of, your Wizard abilities. As a DM, I'd consider giving Sneak attack as written for the Rogue at level 16 to replace the PP Feature, but as written, your next ability is at Level 20, where you can add a level 19 Rogue Daily to your repertoire. An example of which is Snake's Retreat, which would allow you to deal damage once, and then slip away whenever the enemy attempts to assault you. This is all in addition to your continued option to replace Wizard powers with Rogue powers of equal level.
...so basically, I'm not seeing how you're stuck without the ability to play an Arcane Trickster. If that isn't enough, design a Paragon Path for it, or wait until WotC makes one. I'm sure cross-class PPs will crop up eventually.
As for your question about the optimal starting level... 1st should suffice. You're not dirt farmer weak, like it 3.5, and although you certainly get more options at higher levels, you're not in any means limited. Let them get used to 4E's powers and such one level at a time.
