Warriors may be linear and Casters may be quadratic, but...
In 3.5, that doesn’t mean that casters start weaker than mundanes. They don’t. In the situation that most favors mundanes, a straight-up duel at close range at low-level, an archivist or druid can have a significant statistical advantage. This is much closer in balance than higher levels (when the casters can guarantee that the mundane won’t even be able to touch them), but it’s still in the casters’ favor. And they are still far more capable in other situations.
The reason for this, of course, is the judicious application of spells. At level 3, glitterdust might just be the single most effective action you can possibly take. Entangle and grease aren’t far behind.
Druid: Buff and battlefield control
The druid has the easier time. His animal companion can easily keep up with or even best a low-optimization fighter (and therefore using his effectively can reduce the gap between him and the mundanes, even if they are too optimized for the animal companion to actually be equal to them). He has powerful buffs that can be applied to the animal companion (or the fighter, for that matter), excellent battlefield control (such as the aforementioned entangle), and he’s a bit hardier than the archivist.
Archivist: much the same, try to leverage the archivist’s ability to use others’ spells
The archivist has a slightly harder time. His native spells, from the cleric list, are best used for self-buffing and then going into combat; until he gets divine power though, his chassis will hold him back there. Those buffs he can apply to others, however, are his best bets. Though, if he wants, the Zen Archery (Complete Warrior) feat can allow him to use Wisdom on attacks with a bow; combined with divine favor, later divine power, and Dark Knowledge, the “Archervist” can be a pretty solid build. He probably hasn’t taken that feat, though.
Remind (or inform) your casters that characters may work together on crafting magic items: the archivist can supply the gold, XP, and Scribe Scroll feat, while the druid supplies a spell. This allows the archivist to scribe the druid’s spells into his prayerbook, which means he has easy access to, say, the excellent entangle (no, I will not shut up about that spell). It might not be a great choice now, but once he gets divine power, grabbing shillelagh from the druid may also be a good choice.
Also remember that, as a DM, you can drop scrolls for him: if it’s divine, it can go in his prayerbook. You can even have divine scrolls of spells not normally found outside arcane spell lists (glitterdust, for instance), which are always a huge boon to archivists. Giving him spells in this manner is also an excellent way to nudge him towards more effective spells. But be careful; too much of this and the archivist becomes pretty clearly the best class in the game, having access to all the spells.
What spells to use
The biggest thing about spell selection that these two should learn, however, is that HP is not their game. They shouldn’t focus much on either reducing or increasing things’ HP totals. Direct-damage spells tend to be inefficient (especially compared to optimized martial characters), and healing spells tend to be even more so, at least in combat. Sneak a wand of lesser vigor (Spell Compendium) or cure light wounds into their loot, and either of them will be able to apply emergency healing or healing between combat without preparing spells for that purpose. This is good because neither of them gets the cleric’s spontaneous cures,1 and most of the time it is desirable to avoid these spells.
Which brings me to Spell Slots
In surprisingly few levels, spell slots will largely cease to be a major concern. For now, though, the archivist and druid will have to use their spells sparingly: their spells per day are quite limited. That’s OK though: one spell can easily turn the tide of low-level battles. They should get comfortable with the idea that they’ll cast one spell at the start of a fight, and that it should be enough even if the battle lingers thereafter. Potshots with crossbows or what have you are acceptable, just for something to do, but the first spell, most of the time, should have done the trick. Orisons can be used pretty freely, I suppose.
Luckily, both classes have other class features that are actually useful. Just attacking with the animal companion can easily be a fulfilling “turn” for the druid, and Dark Knowledge is a pretty solid buff all around. Dark Knowledge is limited, unlike the animal companion, but it adds a few more things per day he can do, which should pad out his spells nicely. I have sometimes considered a houserule that allows an archivist to apply the bonuses of Knowledge Devotion (Complete Champion) to his teammates, for a kind of knowledge-based version of Inspire Courage. I like the idea, but ultimately the archivist is so powerful that I have never actually implemented it.
And don’t forget to let the mundanes shine
Keeping up with the casters is going to get harder and harder for them. They may be heavily optimized now, but they’ll need to really push it if the casters start picking up a few tricks. At level 3, that’s do-able. By level 7, it’s hard. As the spell levels increase thereafter, it’s looking more and more impossible. That’s just the nature of the game. So don’t entirely take away this time from the mundanes.
1 By the way, summon nature’s ally is pretty solid as a spell, but the 1 round casting time makes it difficult to use and the 3 round duration (at this level) is a big problem. Those spells will come more into their own in a few levels; for now they’re kind of dubious. Rods of Extend Spell and Rapid Spell go a very long way to fixing these problems, though.