Ouch! Lets start the long sad story now...
First you need to check if the power supply and case are ATX style vs AT style. AT has the power cord running up to the front panel. ATX the power cord plugs into the power supply rear. If it is an AT case you need to buy a new case and power supply or start looking for some really old MB's.
1) If the power supply is less than 300W there may be a problem. 250W may work, 200W won't be enough. Also if the power supply is too old it will have 2 seperate connectors that go side by side instead of 1-20 pin double row connector that the new MBs require.
2) Next issue is cooling. If the case is a mini ATX tower then the Power Supply may sit right over the processor. If it does the larger heatsink/fan required for a K7 (Duron/Athlon/Sempron) may be pushed right up against the power supply and not get enough air.
3) MB rear access. The newer cases have a large pop out adapter plate to make the case holes match with various MB connectors. Make sure your case has this. If not you have to find an old style MB which could get difficult.
On the bright side the mounting hole patterns on most MBs are universal now and even the cheapest MB supports USB ports, LAN and on board sound.
As for what MB to recommend.
-What speed CPU are you looking at and how much do you want to spend?
-Do you plan on future upgrades as your daughter gets older?
-Will gaming ever be an issue?
-Do you want built in video on the MB or do you have a video card that will support at least 800x600 (preferably 1024x768) 16 million color. (The early K6's often did not.) 800x600 is barely usable for web browsing the way they design web pages now.