12626 KERALA EXPRESS from New Delhi to Ernakulam Jn. could be the preferred choice.Takes around 46 hours to cover the distance.Try booking the tickets in 2A or 3A to give them a hassle free experience if traveling by A.C is an option or else one can go for the Sleeper Class.P.S - and as Tejas sir told,you have to go the Tatkal route,no doubt in that. :)