I disagree on primers. The ones that "match" the paint are usually just a thinned version of the paint. I much prefer to "seal" the surface with a specialty primer/sealer such as one of the Zinsser products. I really like the B-I-N, which is shellac based. My paint store was touting 1-2-3 or Cover-Stain. I think that they are all better than the paint manufacturers primer. By the way, shellac will not seal against humidity so you have no worry there and you can tint the Zinsser products. Having kids that have left their marks on your walls indicates that one of these are your best choice.