8yo no BM for a few days, then has bouts of stomach cramps, hard stool, then softer stool. But a bunch in one day. Happens often... Ibs? Mom has ibs

Stomach cramps. Sounds like it is constipation which is very common in 8 y/o rather than IBS. Use peaches, pears, and prunes, eating the fruit and drinking the juice. Plenty of water and perhaps Miralax (polyethylene glycol). If you child has not been seen for this then get a check up first.