Greetings,
First I would like to acknowledge some of the fine suggestions made so far the ones I would note so far are;
Dodo
"As for your problem, I say let the dice fall as they may. Your players have been dazzled by specialized character classes which are, at best, supplemental to the core four and they're finding out that they don't have the tactical skill to make up for the class weaknesses. Let them die if that's how the dice go, and your players will figure out that there is an initial balance in the core game that isn't there with supplemental classes."
Orcus
" i never force them to play what they dont want to play. If they want to be a party of gnome rogue-acrobats, fine. Just make sure to bring extra character sheets :)"
Jeffrey Stop
"If your players are tactically-challenged, explain things in more detail. Let them know that if they do such-and-such, they'll take an AoO. Sure, it's a bit of coddling, but if they're learning, it's a good thing. Coach them; after a fight, talk about what happened and why. Tell them how to improve. These things will make them better players and you a better DM."
Tearlach
"Nothing helps tactics explanations like miniatures. Even dice or a stone to represent characters and grids made out of laminated paper work. You can show the combat, point out errors or things they may need to "think about" before execution."
Now I would offer my suggestion. How about some low level fighter type hireling NPCs. I have used such from time to time as DM. I can add a little flavor and have a voice in the game to offer advice should I find it neccessary.
Well alot off advice has been offfered. I hope you can wade through it all. A last piece of advice I would offer is to be nice to your players, communicate with them and let them know you are in this with them.
Later, Derus
