Sanding old paint? ? ?

A little, but the main thing you want to do is prevent lead-laden dust from coating stuff kids and animals play with or touch. A shop vac with the proper filter attached to the sander helps a lot. So does a spray mister and wotking the material wet (with wet-rated sandpaper, of course.)

But before you go down that road, examine alternatives. Is this a wood porch floor, or poured concrete? If wood, how hard would it be to change out the boards? Labor-intensive either way, but may not be worse than sanding. If concrete, I'd strip the minimum amount possible to get a good bite, and encapsulate with new paint.

aem sends....

Use chemical strippers, And yes , lead dust will be left if you sand. Sounds like a bad job, for everyone.

Spend 8 bucks and buy a test kit. Then if there is no lead you don't have to waste your time and money worrying.

If there is lead, you don't want to cause brain damage to children under 7 or leave lead on your property from the dust. There are many sites on the net that touch on proper removal. Sanding should be done wet (yeah, good luck) so no dust is created. A real PIA to do it right..

Where do I get a kit to test for lead?

Most paint stores sell them. It takes a special filter to contain lead dust (in a mask or a vac), most off the shelf ones will do nothing.

Chances are there is more lead from passing autos already in your yard. the amount (if any) in the floor might be hard to measure. The issue is often overblown, but I do not promote ignoring local laws or the potential for damage.