If I feel my kids aren't challenged enough at school, and I can't afford to send them to private school how can I keep them challenged?

Your child attends school between 6 and 7 hours per day. During this time they engage in learning activity. You can enhance learning at home through additional activities that are fun, engaging and interesting without making it feel too much like school. Playing traditional board games like Scrabble can be fun and educational. Cookery can teach your child about weights and measurements. Throwing, bouncing and catching a ball is a useful way of developing 'hand-eye' co-ordination and motor skills. 

Try not to overwhelm your child with additional academic learning it can become too much for them. Allow them to experience childhood fully and encourage some family fun.

you need to keep your child motivated even after school.... when children do not face competition they tend to take life easy and develop a lay-back attitude....

sometimes motivational interviewing can help a lot and give your child that push to strive more than the mark expected from him/her..

for details on how to carry out motivational interviewing and its benefits visit this link here.