The older children had already experienced how the toddler 'played'. She grabbed. They would know how a toddle behaves, and they are applying that knowledge to this situation. They didn't want the ship grabbed and their game ruined. Children are literal, and often very honest. They didn't hit the toddler, they didn't push her or scream at her. They verbally told her mother their desire to play alone. I think that this in itself shows some very good social skills for young children. I know many 4 or 5 yos who may well have walloped the toddler instead of verbalizing their wishes.
I think the mother of the toddler should then respond and take their wishes seriously. If she felt that they could have expressed it more politely, maybe paraphrasing, such as, "Oh, you'd prefer to play with it on your own?" would make you feel that you'd turned it into a more positive experience all round, followed by something to the toddler like, "OK, honey, these kids want to finish up their game, so let's play with the blocks over here until they are done."
I have had a zillion interactions like this in my time with young children, and see nothing but positive in it, a learning experience for everyone. Later, when the older kids give up on the ship, you can thank them and model good manners again. Which is so very different to giving them a rule, which is by no means a univerally understood one, that they had to let a toddler move in on their game.
I like this reply.