Step parenting is never going to be an easy step.
Handling a child's emotions when taking that step depends on the child, their age , maturity and their character.
As some have noted- some children actually are happy for their single mother/father to be with someone who makes them happy . The older the child and maturer they are- they will realise that you are NOT going to be shutting them out of your life but that you are taking a step that is going to make you happy and it will roll over into their lives - as happiness breeds contentment.
However you will get the child , who regardless of their age ,is not accepting the fact that mum or Dad has found someone new and they will make it as difficult as possible.
For the younger child- they basically need reassurance that they will be a PART of this new family set up- as many times they may have fantasized also about their parent getting back together with their other biological parent. When this hasn't happened they may get very difficult.
LOVE is key and you will need a lot of PATIENCE to tackle this IF you are going to get married. As it wont happen overnight but if you continue to treat them with love they will eventually in time see that they are a part of the new family set up and that the step parent isn't the 'wicked witch of the west' who is there to usurp them out of the family
So its important to make them feel INCLUDED especially if the person you intend to marry has kids also...
There are also many helpful christian books online that can help anyone who are preparing to be a step parent .... I would read up on some of them and get spiritual counselling where necessary in some cases ...as with step parenting comes a lot of issues that needs to be addressed and which the child may need to express in an environment where it is encouraged to do so...
That way a lot of potentially damaging issues can be sorted out BEFORE you walk down the aisle as your child will feel they have been listened to- been heard and their worries and fears acknowledged and addressed. It will help to allay their worries and sometimes calm their fears that they are NOT going to be on the sidelines of this new marriage.
For some children though, they may never be able to accept the fact that you are marrying someone else- even after doing all these steps...so you have to decide whether you wish to take that chance that they will come around some day when the step parent continues to show them love and acceptance and NEVER say that I'm your new mom or Dad- kids hate that as they already have a biological mom or dad they love
Or you give up the idea of marriage all together or put it on hold until they are older .... I have known some people to do this sadly and who lost out on love because their child manipulated them into giving up someone they loved... depends what you wish to do in these cases as a child will eventually grow up and get married some day themselves and you will still be on your own.... so PRAY first and foremost about these issues and ask God to let you choose wisely on this matter.