A few ways that might work
1: Distract them with a toy/stuffed animal.
2: Bribe them.
3: Tell the kid that they will be like all the older kids if they take it. Boosting their ego will help them.
4: Tell them straight out that its good for them.
Remember to always not get frustrated if they refused if you do get frustrated dont let the child see you frustrated.
Im not a parent but these are things that have worked on me/siblings/friends when we were young(er)