Hallooo

Y'all I'm patiently waiting before my comment be buried in history

I assume you already read the popular ones?
https://www.mangago.zone/read-manga/saihate_no_paladin/
https://www.mangago.zone/read-manga/mushoku_tensei_isekai_ittara_honki_dasu/
https://www.mangago.zone/read-manga/kumo_desu_ga_nani_ka/

an ofc not child, but isekai manga, not harem or bl
https://www.mangago.zone/read-manga/tensei_shitara_slime_datta_ken/
https://www.mangago.zone/read-manga/tensei_kenja_no_isekai_raifu_daini_no_shokugyo_wo_ete_sekai_saikyou_ni_narimashita/
https://www.mangago.zone/read-manga/tondemo_skill_de_isekai_hourou_meshi/
https://www.mangago.zone/read-manga/a_mild_noble_s_vacation_suggestion/

I've sadly read them already and also watched the anime of it i wanna discover more huhu

does it have to be manga or is manhwa acceptable?

well it can be it's just that i haven't read a manga like it for so long :(((

I actually have a list of reads like what i wanted but I don't really think I put them all here tho i wanted to read moree of it

https://www.mangago.zone/home/mangalist/2051288/

ok, so you've already read all of what I could recommend lol, so what's left is all that matches only some of your expectations, hope it helps :P

this is the closest:
https://www.mangago.zone/read-manga/jitsu_wa_ore_saikyou_deshita/
https://www.mangago.zone/read-manga/dad_is_a_hero_mom_is_a_spirit_i_m_a_reincarnator/
https://www.mangago.zone/read-manga/kamitachi_ni_hirowareta_otoko/

this also seems to fit, but I don't remember it at all
https://www.mangago.zone/read-manga/isekai_de_mofumofu_nadenade_suru_tame_ni_ganbattemasu/

child MC, isekai, bot no fluff
https://www.mangago.zone/read-manga/yondome_wa_iyana_shi_zokusei_majutsushi/
https://www.mangago.zone/read-manga/akuyaku_reijou_no_ani_ni_tensei_shimashita/
https://www.mangago.zone/read-manga/suddenly_became_a_princess_one_day/
https://www.mangago.zone/read-manga/daughter_of_the_emperor/
https://www.mangago.zone/read-manga/even_if_the_villain_s_daughter_regresses_promo/
https://www.mangago.zone/read-manga/born_as_the_daughter_of_the_wicked_woman/
https://www.mangago.zone/read-manga/kekkaishi_e_no_tensei/
https://www.mangago.zone/read-manga/reincarnated_as_an_aristocrat_with_an_appraisal_skill/
https://www.mangago.zone/read-manga/isekai_yakkyoku/
https://www.mangago.zone/read-manga/i_will_leisurely_become_a_healer_in_another_world/
https://www.mangago.zone/read-manga/the_youngest_princess/
https://www.mangago.zone/read-manga/i_was_born_as_the_demon_lord_s_daughter/
https://www.mangago.zone/read-manga/shindou_sefiria_no_gekokujou_program/
https://www.mangago.zone/read-manga/zennin_ossan_umarekawattara_sss_rank_jinsei_ga_kakutei_shita/

starts as a child, but has a time skip to teen
https://www.mangago.zone/read-manga/i_will_remove_them_from_my_life/

child mc, regression
https://www.mangago.zone/read-manga/a_saintess_who_was_adopted_by_the_grand_duke/
https://www.mangago.zone/read-manga/becoming_the_villain_s_family/

not exactly a child, but a teen
https://www.mangago.zone/read-manga/i_became_the_daughter_in_law_of_the_villain_because_i_m_terminally_ill/
https://www.mangago.zone/read-manga/how_a_former_pro_takes_advantage_by_doing_an_easy_job/
https://www.mangago.zone/read-manga/the_perks_of_being_a_villain/
https://www.mangago.zone/read-manga/isekai_walking/
https://www.mangago.zone/read-manga/saikyou_no_shokugyou_wa_yuusha_demo_kenja_demo_naku_kanteishi_kari_rashii_desu_yo/
https://www.mangago.zone/read-manga/kumo_desu_ga_nani_ka/

isekai and fluffs, but not child mc
https://www.mangago.zone/read-manga/healing_life_through_camping_in_another_world/
https://www.mangago.zone/read-manga/tondemo_skill_de_isekai_hourou_meshi/
https://www.mangago.zone/read-manga/cheat_pharmacist_s_slow_life_making_a_drugstore_in_another_world/

honourable mentions:
teen mc, not isekai, but fantasy setting
https://www.mangago.zone/read-manga/angel_of_the_golden_aura/

child mc, fantasy setting, but not isekai, very adorable
https://www.mangago.zone/read-manga/taimashi_to_akuma_chan/

I really don't remember reading this, but it's on my "read" list and has a child in the thumbnail (≧∀≦)
https://www.mangago.zone/read-manga/dungeon_nursery/

Omagahh i love the effort thank you so mcuh