They used to be among the top search results on NHentai.net if you searched "MILF + Shota" and "DILF + Loli", but now they seem to be impossible to find again, so I don't know if NHentai removed them already.

1) The first is a series where a single father and his son secretly get into romantic/sexual relationships with their next-door neighbors in their apartment complex who are a single mother and her daughter. The single parents were childhood friends who either used to date each other when they were young or who friendzoned each other , I cannot remember exactly , and both couples are shocked when they discover that they are actually secretly dating each other's parents/children. This is roughly the same exact plot as another vanilla hentai, the one-shot "Lovers Under The Same Roof", which was indeed mentioned in the comments section, with a link to it also on NHentai.

2) The second is a series where a young teenage boy secretly gets into a romantic/sexual relationship with the single mother of his older brother's same-age girlfriend, so while his brother and his brother's girlfriend are frolicking together on the beach in a joint summer outing, the younger brother is actually having sex elsewhere on the beach with the girlfriend's mother, who is a Western-looking MILF with long curly blonde hair. This is roughly the same exact plot as the "Summer Time" series, except that the blonde MILF in Summer Time is an actual Western foreign tourist staying at the Japanese boy's family inn, and she has no children of her own yet.