Ezlink card expires 4 years after purchase (by right). If you son is less than 0.9m then he can travel for free, if he is between 0.9m and 1.2m, he will pay child fare. However, take note that if he is above 1.2m(That is if your son is real tall), he will have to pay FULL adult fare (Concession applies to Singapore Students only).
I would advise that you let your son keep that card and forget about getting the $1 deposit upon returning but you should refund your card and get the $3 deposit and take note that there is a $5 Non - refundable deposit for each card.
Using Ezlink ensures you rebated transfers between MRT and bus and vice versa. However, you may wish to take note of the Child Ezlink card serial number at the back of the card in an event that he looses it other people may not be able to use his card (Adults dont enjoy this).
MRT fares for children are 40 cents/45 cents and 50 cents for NON transfer from buses but transfering from buses would be a 10 cent rebate if i could stil remember and adults also do enjoy rebate while transfering from bus.
If it happens that your child dint manage to get a ezlink card, then he would have to pay full adult cash fare (No concession/child fare 'cash' fare available <_<) and the standard ticket also comes with $1 deposit (Refundable, same for adult and children) and buying a standard ticket does take up some time as you would have to select station, insert money and so on and quite a hassle having to refund the card everytime unless you wish to keep it and the expense of $1.