Can children challenge a marital trust that I made in favor of a second wife?

Possibly. How easy a challenge would be to bring, and how likely to succeed it might be, depends on a whole lot of different factors. For example, a trust created for your second wife under your Will, which will have to be funded with probate assets after your death, is more susceptible to a challenge than an irrevocable trust which was fully funded by you during your lifetime and already exists. If you used a competent estate planning attorney, then you and the attorney should have discussed those factors and decided on a plan which met your goals in the manner which made you most comfortable. If you didn't use an estate planning attorney, or if your attorney really focuses on areas of law other than estate planning, then you may want to get a second opinion from an attorney who does focus primarily on estate planning.