Does anyone have cute stories that happens in another world and mc is a kid? Anything like...

If you search isekai and in the cover there's a girl/child, it is almost 80% guaranteed that the story is all about fluff, family affection and spoiling children. ╮(￣▽￣)╭
http://www.mangago.zone/tag/Isekai/

But if I have to recommend some that is not about girls desperate for fathers love and is not focused on noble girls shoujo but adventure and humour I recommend:
http://www.mangago.zone/read-manga/i_will_leisurely_become_a_healer_in_another_world/
http://www.mangago.zone/read-manga/otome_game_no_hametsu_flag_shika_nai_akuyaku_reijou_ni_tensei_shite_shimatta/
This one is set in another world, protagonist start off as a child:
http://www.mangago.zone/read-manga/i_want_to_be_a_receptionist_of_the_magic_world/

This one is a novel but definitely one of the most funny story that start of with child. First part of the story is set during elementary till highschool. Simple fluff, comedy and Japanese life story about villainess and love interest childhoods before the heroine's arrival:
https://www.novelupdates.com/series/kenkyo-kenjitsu-o-motto-ni-ikite-orimasu/

Since you seem to have enjoyed that story with slimes I recommend this. I may look like smut or the usual harem story but no this story is built on adventure rather than romance.
http://www.mangago.zone/read-manga/tensei_shitara_slime_datta_ken/?lX8x

Thanks a lot !