Just came across this topic.
I have a now almost 2 year old son and have already printed quite a bit of toys for him of my own design. Mainly some toy cars (in PETG and PLA) and a walking bike.
I’ve put a number of the earlier design on Thingiverse, like these:
One thing though, you should not make any parts too small and delicate. My son is able to destroy anything that’s not really strong. That goes not only for the things I’ve made but also about 80% of all the toys he got, so even the factory made stuff often does not last long