tilly's advice is very good. Probably a neighbor or two is the same straits and you can get organized about it so you each end up needing to pay for child care less frequently.

A good nanny or sitter can be had (if you carefully vet) for as low as $10-18/hour; This site is worth the subscription if you're in a location it covers: Sittercity. Bite the bullet and get one and look to extended family (if you carefully vet) for more help. An au pair is an option too, especially for a single parent. It ends up costing $15-$20K for a year for dedicated full time child care and can be very good for a family because of the cultural exchange and fresh outlook.

Working with kids underfoot is a mistake; it creates an environment wherein you can, almost certainly will, get overly crabby with them because you have a boss or client barking at you over a production bug or a deadline while your child is trying to get attention any way possible.

When it just doesn't work out for a given day there are lots of great activity books out there (e.g., McNally has a nice travel/activity book series which can be very engaging to the 4+ set). There are also DVD sets from Noggin (Backyardigans, Little Bill, and Lazy Town are some of the better series for kids; Dora and a few others are becoming a sort of Pokemon-lite to push merchandise) and such which you can rip to put on an iPod Nano (which has a master/lockable volume setting to protect hearing; set it at 50% and set the lock code). That way you can have the kid in the room with you, safely in view, silently engaged for a couple of hours.

By the age of 5 some kids are quite ready for a Nintendo DS or a Wii and they have a handful of games that are just right for that age set (Animal Genius for the non-reader comes to mind). I'm personally acquainted with a 6 year-old girl who dusted all the high scores on a non-kid PS2 game recently.

Good luck. Don't get too frustrated. Get organized. Realize you'll have difficult days. Take care of yourself and your attitude. Work is important. Happy, well-attended kids are moreso.