What is the Ruling on Giving Gifts to Encourage Children to Pray?

There is no problem with that if it is not a plan to favour some of them over the others. Rather, it is only a means to encourage them towards goodness. And this is mostly something that is little and overlooked. Because this is from the means of encouraging and aiding in goodness. But if they are gifts with value and some are preferred over the others, then no. However, if it is something small for encouraging then no problem, if Allah wills.