Hi there
I had to collect a urine sample from my LO when she was about 7 weeks old, and again only a couple of months ago when she was just over 2 (still in nappies). Both times, I was given a pack from our GP which contained 2 sterile pads (like sanitary towels) which are slow at absorbing urine, a syringe and a sample pot. The idea is to place one of the pads inside the nappy, and keep checking it every 10 minutes or so. When the pad is wet, remove it from the nappy and use the syringe at an angle to draw out the urine (this can take a few times to get a good amount!) and transfer into the sample pot.
It's actually easier than it sounds, but the hassle is keep checking the nappy until they have wee'd and also, if they soil the nappy, you have to start again!
So it may be worth asking if your GP have these packs
HTH
Jen x