I moved breakables up high and left the bottom drawers nearly empty and put stuff in there to play with With my eldest two I never placed drawer locks or stair gate. But with my youngest every protection and more is needed even now she’s 5yrs old

Rather than buying draw locks just for temporary use, you can tie all the draw handles together with some string/the tie from your dressing gown. It means they can’t get into the bottom draw so you done have to empty it also kids like to open drawers on chest of drawers and try climb them which can make them topple on children so just keep an eye out around chest of drawers/bookcases etc. and large flat screen tv’s can fall forward on them so either push back out of reach or you can get a cheap safety strap which straps the back of the tv to your tv unit so it can’t fall forward this all depends on how long you are having your niece. If it’s for a short time just keep an eye on all these things

Firstly, start with fixing the most obvious problems. Moving items to higher shelves, blockading the heater, getting power point covers, blind cords etc The rest you will have to figure out as you go along. See what she gets into and remove it