So my top tips here would be:
1) Work out what it is exactly that you need ie: a wardrobe, desk, chest of drawers.
2) Work out how much space you have to play with
3) If you haven’t got a piece of old furniture that fits the job, ask around friends, family might. If they don’t get onto Ebay or FB market place and specifically search for what you want.
4) Make sure you get the correct paint for the job and always check drying times between coats and if you need a primer or not. Trust me, a little preparation up front prevents a lot of mistakes and upcycling disasters!
5) Check-put Etsy for some unique and quirky handles or knobs.
6) Let your imagination run wild! After all it’s a child’s bedroom, let it be fun and creative.