Welcome back to D&D. Enjoy.
Also, the essentials druid has a leader build, The Sentinel.
Later on, you'll want magic items that can maximize the healing surges you use, like the Cloak of the Walking Wounded or the Amulet of Life.
Power 
Daily (Standard Action)
When you use the exodus knife to trace a doorway onto a solid object, it opens a portal into an empty extradimensional space 4 squares wide, 4 squares high, and 4 squares long.
While the door is open, anyone can enter, see into, or affect the extradimensional space. Only creatures inside the space can open or close the door (a minor action). Once closed, the door becomes invisible to anyone outside the extradimensional space. Creatures on the inside of the closed door can see out, but those outside can't see in. Creatures on one side of the closed door cannot affect creatures on the other side.
The extradimensional space lasts for 8 hours. Any creatures still in the space when the effect ends reappear in the closest unoccupied squares outside the door.
I would probably do the same thing again since it worked out so well and let them play the classes they wanted.
Be mindful of these problems, be ESPECIALLY wary of lurkers, and you should be fine.
-SYB
Unlike previous editions, you don't need a healer/leader to survive.
Actually I disagree. Out of the box the healer is needed just as much as in any other edition.
Sure, you can adjust your game to work without a leader, but that requires the same work from DM-side and player-side than in any previous edition.
4e's promise that you "no longer need the cleric", just means that you can take the warlord or shaman and be just as good.
Yet going leaderless requires just as much special care as before
Yet going leaderless requires just as much special care as before
So does going defender-less, striker-less, or controller-less, though. If you have a huge gaping hole in your party make-up somewhere, you need to take a few steps to mitigate it, or something will come and punch you in it.
It's perfectly doable though. Especially if the Barbarian is willing to go Thaneborn and the Warden (that'd be you, OP) goes for Life Warden. That should give you enough leadery-ness to survive, especially combined with a few healing potions and some careful tactics.
Also, what others have said about taking MC feats or other options to create healing opportunities.
...that's what gamma world did.
You don't need any one class to be successful, just need to know the party's strengths and weaknesses....though be prepared for random death (still pretty hard to die in most cases), as the rolls will go against the characters at some point.
Unlike previous editions, you don't need a healer/leader to survive. You just need to take that lack into account and design your group tactics around it. For example, since you won't have someone handing out attack bonuses, you'll need to do your best to flank at every opportunity to improve your chances to hit.
Later on, you'll want magic items that can maximize the healing surges you use, like the Cloak of the Walking Wounded or the Amulet of Life.
What are you talking about? I play 3.5, playing without a healer is just fine, especially because if you do have a "healer", their healing spells end up healing far less damage than a different spell fo the same level could prevent!(for instance, the druid could heal 1d8+1 or entangle a bunch of the enemy fighters... a no-brainer really, especially if you consider that by the time you're rolling 3d8 for healing, the enemies deal -far- more damage than that in an equivalent action)
Healing in 3.5 was best left out of combat. A wand of Vigor, lesser will carry your party through the first 550 damage inflicted upon them. When it runs out, invest in some healing belts/2 new wands!
Rolling a Cleric, thinking you'll be your God's hand of justice on this plane, and then being relegated to healing duty because nobody grasps that there are better, cheap solutions out there... Well, let's just say that out of all the worst things possible that is THE. WORST. POSSIBLE. THING! ;)
You can have a party without a healer, have fun and kick tail at the same time 
One of the groups I was in had a clericless party for a good while (two fighter rouges with spiked chains and a barbarian) we were an absolute menace to our foes. We also had to make extensive use of fighting defensively, the expertise feat, fighting smart and probably independently supported the city's temples without healing potion purchases. But it is very doable.
If the DM throws killer encounters at you, yea, you will probably die.
But the DM should be having slightly weaker groups of enemies, and or let you rest more often.
Once I allowed second wind to be used twice per encounter for a leaderless party, that helped.
A party with no leader requires the DM to adjust a bit, just like a party of eight with four leaders would require the DM to adjust.
just like a party of eight with four leaders would require the DM to adjust.
If they are four warlords with different Presences, he'll probably just double the combat encounters.