It seems to me that Louise assumed that having another girl (and let's be real she was RELIEVED Pearl was a girl, I don't think she thought she'd know what to do with a boy) that Pearl would be exactly the same as Darcy.
She's shown enough of Darcy that we know that she's a very people pleasing, sweet girl, intelligent, likes reading and other calm, creative activities, like sitting in dens, and playing with small toys like her potions, and her dolls and sylvanian families. She clearly has a lot of imagination. That's what Louise is used to, and also what she seems to be into, crafty type activities.
Louise said herself at the beginning of lockdown that Pearl needs to run and climb. She's shown her in the soft play just going down huge slides with no fear while Louise is at the top freaking out, she's said multiple times that Pearl has a lot of energy and needs an outlet for it.
Then why, in all her 'insta perfect' lockdown footage on instagram, do we never see activities tailored to Pearl's interests? It's always crafty things with Darcy and Pearl is sat there joining in as best she can, where's the paddling pool where she can scream and splash around? Even the climbing frame Louise bought, she made it into a 'den' instead of getting one that's safe for Pearl to climb all over, or maybe getting a trampoline. The only activity Pearl seems to get to do to let her energy out is riding a tricycle round and round her stupid little garden path. It seems to me that Louise has this set idea of what girls are like and what girls like to play with, when maybe Pearl might enjoy stuff that's a bit rougher and more adventurous.