I’ve tried various ones for my 5 year old and have come up with a combination that really works for him.
For the elbows he’s using Fox PeeWee. These are actually supposed to be a motocross product but they are perfect for everything, they strike a great balance between being protective but also very light and comfy.
For the knees he’s using G-Form pads. These are great for kids as they are so light and unobtrusive, but as mentioned above they are expensive.
Top tip for these is that all the models (elbow/knee/shin) are very similar and can be used for different purposes so shop around. I bought my son the shin pads in medium size which work perfectly as a knee pad for him and were reduced from £45 to £22 at SkateHut.co.uk