Haven't done the sahara yet with kids - it's on the agenda! - but we've been to some pretty out-of-the-way places with young kids, so maybe some of the below will help. We were in morocco (in and around marrakech) when our daughter was about 2 1/2, and she LOVED it. Have fun!
(i) Bring some rehydration solution - more than you think you'll need (here in france they sell it in powdered form, that you can mix with bottled water, which saves on space). Kids can dehydrate very quickly with traveller's tummy, or even car sickness, and heat only adds to that. I think that in a pinch you can make your own, but the powdered/pre-made stuff has everything in the right proportions.
(ii) ditto on the baby tylenol - you need something to get a fever down quickly in an emergency (and, touch wood, it's kind of like an umbrella - if you bring it, you're less likely to need it!)
(iii) an extra package of diaper wipes can do double-duty as clean-up wipes, since kids touch things and put their fingers in their mouths a lot (but note that at least a little exposure to germs is a *good* thing and may make them less prone to travel tummy later in life ...)
(iv) you might want to stock up on diapers in the larger towns - some of the more out of the way villages may not have them, or have only the small sizes. Ditto for formula if you're still using it.
(v) we found that it was difficult to get sunscreen in some places, particularly in more remote towns in africa. Babies and toddlers (if they're light-skinned) burn very easily, so it's best to bring plenty along just in case. Ditto for a hat and sunglasses, for the same reason.
(vi) how's your child with new foods? ours both love rice (and olives), so always could find something to eat - but you may want to bring along some multivitamins just in case he/she gets stuck in a "rice and only rice" rut for a few weeks on end.
(vii) I think most of the travel vaccinations can be given to a kid who is 18 months - check with your local travel clinic and find out what you need and what can be given
(viii) if you're hitting any mozzy zones, there's a new-ish antimalarial out, called Malarone, that comes in a pediatric formula that is prescribed by weight. The pills taste really nasty if the kid holds them in their mouth too long (coating dissolves off) - ours were really resistant until we super-coated the pill in a bit of extra starburst candy as a coating, and then it slid down fine (the candy gave enough extra time for it to be swallowed without disolving). The first mozzy-zone trip (at 18 months, pre-Malarone), our ped said just to grind the pill up and put it in applesauce, but that didn't work; just resulted in our kid refusing to eat applesauce for about 6 months! I've heard of parents who had success grinding the pills into a spoonful of syrup or something else super-sweet, though, so if your kid isn't into swallowing chunky stuff yet you might give that a try.
(ix) kids tend to go through at least twice as many clothes while travelling as you'd expect an adult to ...
(x) rather than lots of toys, we tend to travel with a couple of stuffed animals who can do double-duty as puppets/companions/narrators/etc., along with lots of paper and colored pencils and several familiar and unfamiliar books. If you go the colored pencil route, a small pencil sharpener is always handy to have ...
(xi) sturdy shoes for the little tyke are a must - and yeah, they make cute little hiking boots for kids (grin)
(xii) a backpack carrier is really nice to have if you're intending to go on any hikes, or otherwise get away from your vehicle much. We've never travelled with a stroller, though we know others who swear by them - kind of a personal preference thing
(xiii) I'm presuming you're in your own vehicle, and strapping in to a car seat from the get-go. If you're flying and renting instead, then check with the company beforehand - some of the rentals may not have seatbelts in the back, in which case you'll either need to argue or jerry-rig something.
(xiv) all kids are different, so you may find that none of the above applies!