I quite like dunkman's suggestion to try out the $15 plan for young children because you will receive free data and minutes this month as well and it gives you both time to see how much usage would be needed. And it doesn't cost extra to change plans though most of the posters do recommend to change at next renewal since prepaid service means no refunds.
The only cheaper plan is the $15 plan for 250 mb data and 100 Canada outgoing minutes. This is sufficient for some kids.
If you activate before December 31, you will also get holiday data bonus and talk bonus add ons, you could activate the $15 plan cheaper plan for 1-2 months to use up these bonuses. Then if your son needs more data, switch to $25 plan.
Otherwise, the cheaper plan usually don't have too many deals.